Sometimes I feel Smash fans act as if every single player buys every single DLC character and would continue to do so indefinitely...
Look, it's one thing to love DLC and the nature of content it enables, but you gotta admit it's terrible value relative to how much you get for your money in the base game, and generally diminishing returns for the company the longer it goes. Those who still can't fathom how Nintendo could possibly decide against doing a third Fighter Pass may need to wake up.
Not to mention how a potential evergreen system-seller like Smash desperately needs to be up-to-date mechanically and graphically every gen. Backwards compatibility of the Wii and Wii U did not render Brawl or Smash 4 unnecessary.
Continued DLC or DLC-styled content packs are simply not sustainable as the primary or premier source of new content for Smash. It's not the absolute win-win for player and developer that some make it out to be. Besides, even if I'm totally off and Smash could make it work for another whole gen somehow, we'll simply wind up in the same situation again, only worse and worse as far as the possibility of porting everything over and retaining the roster goes.
I have literally never seen a fanbase with so much straight-up animosity towards the very idea of a brand-new entry in their series. At a certain point, it's too much in my opinion. Regardless of your stance on particular characters, this overarching "no cuts, no buts" attitude is just not the hill I'm gonna die on when it comes to the longterm well-being of our beloved franchise.
I'd say a third Fighter Pass has been out of the question for a while now.
I wasn't exactly thinking about endless DLC, it's more that (in my opinion) after the jump from Melee to Brawl the series has been very iterative. Not only characters, but also stages and items are quite likely to stay in the series once they've been introduced. Even Smash 4 reused a lot of stages from the previous games, though not to the extent of Ultimate.
In short, this series has a tendency to add add add in terms of the core content - what actually comes and goes for real is usually the single player modes and the collectibles.
Don't live service games often do jumps like that from Smash 4 to Ultimate? Tweak some mechanics, rework some characters, introduce some new mode... DLC introducing new single player modes in particular, as opposed to just characters, I think would go over well with more casual players.
I'm not sure if it's sustainable, but the way I see it, fans clearly love the crazy amount of content in these games, the developers like iteration (probably as a response to the fans liking it), and during the Ultimate DLC half the industry seemed very eager to jump onto the Smash train in some form - be it through a full-fledged character, a Mii costume or even just a batch of Spirits.
Resetting everything just seems... weird.
A normal sequel that loses some old content in favor of new one is pretty reasonable, though. But I wonder how exciting that's going to be unless they introduce something really crazy. Some form of service Smash seems like the safest route to surpass Ultimate.