The future lifespan of smash bros is kind of a unique case unto itself. There's only so much you can do to improve the graphics and mechanics with such a franchise without turning it into something significantly different. That said, I seriously doubt Nintendo will be able to, or even want to, sell another iteration of the game on the switch. Why? Think of what they could even use for a selling point. More characters? Sakurai himself admitted the difficulty of such a full roster. We'd be lucky if the next game were able to match the sheer quantity of characters assuming we don't lose a big part of the roster in exchange for a bunch of new ones. And even if by some miracle we got EVERY SINGLE EXISTING CHARACTER plus a bunch of new ones, why couldn't they have just made the new ones DLC? The only way to make the new game stand apart would be the addition of some seriously awesome modes or new mechanics, but judging by ultimate, I seriously doubt we can have our cake and eat it too with a roster that huge AND have room for all those extras. The software/hardware has limits, not to mention the health of the devs lol.
Nintendo tends to release one smash per console for this reason. You need some sort of significant addition to incentivize players to upgrade. That said, Nintendo also wants to milk the switch's life for as long as they can, thus keeping Ultimate relevant for as long as they can kills two birds with one stone. How do you keep Ultimate relevant? DLC, baby. We can rationalize all we want about what Nintendo will end up doing, but at the end of the day, smash DLC=easy money, plain and simple. I don't see the 15 slots as unreasonable. Lofty, perhaps, but not unreasonable. Obviously, the prospect of more DLC packs isn't definitive, but it would make a lot of sense. I also wouldn't assume that Sakurai will be the end all of Smash bros. He's hella critical, that's for sure, but if he keeled over tomorrow, do you honestly think Nintendo would just pack it up? You think they'd just sit on a billion dollar franchise because its George Lucas checked out? Hell nah. Even if they couldn't match his skills, they'd still try.
To be clear, I certainly don't think Sakurai's exit from Smash will mean the end of Smash. I just completely doubt that they would replace him during the DLC cycle of the game he developed because he wields so much power and devoted so much passion to the project. Unless the new head of NOJ is a real piece of **** who will pull some Konami and Kojima The Phantom Pain era bull****, when Sakurai says he's done, he's done and so is Smash Ultimate.
We also need to remember characters aren't the only major content for this game and that things like Stage Builder and Video Editor are big free additions that make the game more viable for the future. There's a theoretically near endless number of stages that will be made for the game and people can use the Editor to make their own unique content for years to come. Content like that adds replay value by player creativity and thus adds longer legs to the game. Not to mention we may see additional modes like Home Run Contest come in future content updates. Such decisions help make the game a more enticing purchase to consumers for years to come while the Switch is still relevant.
Also, I don't think it's worth bringing up the question of where Smash goes from here as anything relevant to DLC. That's a question for the next Smash game and whatever that entails. There are a variety of interesting questions to pose for the future of Smash most certainly, but I suspect we will not break our patterns of one Smash game per console. Nintendo and other companies always talk a big game of how long they want their consoles to last, but the Switch is likely to see a successor within the next four years. The system already falls a generation behind current gen consoles like the PS4 and the Xbox One, when the successors to those consoles release, Nintendo's looking at an even more uphill battle for content, especially when so many developers have already made Switch titles. Yes the Switch will sale as long as it's successful and people are willing to invest in games for the console, but that will only become more difficult. If you go by a more conventional six year cycle of a console, Smash released a year and a half into the console's life cycle, and the first set of DLC with be completed by the end of the third year. Given that Nintendo has not particularly committed to the games as a service model yet (They have made some forays into that arena, but we're still a long way removed from your Sieges, Destiny's, and so on), I think that fits their operating status since only Splatoon 2 has had legs of even comparable length.
I'll continue to also push back on Ultimate needs DLC to be relevant forever. It's an evergeen sales title that has shown an absurd retention rate with Switch consumers. Selling 13 million copies in 4 months is nothing to scoff at, and while those sales will not be indicative of consistently amazing sales, it's safe to say Smash will likely follow the precedents set by the likes of Breath of the Wild, Splatoon, and Super Mario Odyssey and continue selling on name recognition alone. Sure, DLC can potentially increase sales and bring in more revenue, but there's also an economic advantage to not consistently developing DLC. If the game serves as an evergreen title that continues to sale on its own and also sales the initial Fighter's Pass, then they have a revenue source already. They don't need to invest in developing more content and devoting more resources to the project when they already have a cash flow. I'm not saying that's the only sensible decision, but it's definitely
a sensible option. The Fighter's Pass will continue to create hype and I'm sure characters will always create additional buzz for the game. But, everything has a natural end point, and if Sakurai says he's done, Nintendo more than likely will respect his wishes since he's already created such a major cash flow for them anyway.
Again, all I'm really saying is that it depends on Sakurai. His decision making determines everything for Ultimate, and I think he's a much less clear individual to read in this specific situation.
Honestly, they shouldn't make another Smash for a good while.
It's clear how much hell Sakurai went through to even get Ultimate's base game complete. He's expressed himself how much easier DLC is on him, as well, and how he wants to do as much as he can for Ultimate. Nintendo wants Ultimate to sell for a long time and to appeal to everyone, both new audiences and old fans. When people say they want Sakurai to take a huge break, too, I don't think they realize how implausible that is. Japanese work force culture is stringent, and Sakurai's hard-working demeanor is a product of it. Nintendo won't let him take a break, nor will he allow himself to. Trust me, I wish Sakurai could just take a step back, but it's ridiculously rough overseas. It's nothing like the West.
The best, happiest medium would be just to support Ultimate with DLC for a long time. Sakurai gets to take it easy as much as he's allowed and everyone gets more content for an already amazing game. It's win-win. Of course, Nintendo is daft as can be, so we don't know if they'll actually do that.
Japanese work culture is stringent and Sakurai applies himself to that, but he has mentioned taking short breaks before and he has tended to work on other projects in between big Smash releases. Ultimate is the first time since 64 and Melee that he has gone straight into another Smash game after completing the last one. It's difficult to read where he is in this situation, hence my comments above, but I wouldn't be surprised at all if he decides to finish his work on Smash Ultimate with the end of the Fighter's Pass and move on to other projects. He's a talented developer with lots of unique ideas, and he is likely to basically have a blank check when it comes to developing most of the ideas for Nintendo.
Ultimate already encompasses the culmination of his work on Smash. It will serve as an absolutely amazing and perfect note to end his current Smash status (or even entire career) on regardless of when he chooses to do that. It would have served as such without the Fighter's Pass, so I still consider us somewhat lucky that we're getting so much post-release content as is. And after a year a few months of additional support, that legacy will only be further cemented.
Sakurai may be the product of a grueling systems of work ethic and responsibility, but don't underestimate just how much power he wields in comparison to most other employees and workers in Japan. That power gives him several more options than your average employee or even employer, and probably several more that we're completely unaware of.