A few things to comment on:
1) I'm not convinced Smash will be announced again in a few years. I think anybody would find that as a surprise at this point. While I can agree that Smash is a system seller, the same could be said for Mario Kart, yet we haven't seen a new Mario Kart since Deluxe was ported to the Switch at launch...and Mario Kart makes more money than Smash by a LONG shot. Based on the JP Nintendo website, which I just looked up, MK8 Deluxe has sold 37.8 million pcs, while Smash has sold 24.77 million pcs. Neither is a sad number, but the difference is still astonishing.
Basically, I can't disagree it's a system seller. Yes, Mario Kart 8 is a port of the previous Gen Mario Kart, and at the same token a “new” Mario Kart. But that logic does not necessarily suggest they're going to jump right back in and make an entirely new Smash game off the heels of one of the biggest and most expensive projects they have undertaken. To me, this feels different just in the virtue that this game simply has been a decade in the making essentially. There are other questions that need answering too. Is Sakurai working on a different project now? Is he retiring? Will somebody else take over Smash? There are larger questions to answer than just, "When new Smash?"
I also think folks are underestimating what next steps are. In terms of what Smash is now, the toothpaste is going to be quite difficult to put back into the tube. You've maximized fan demand by adding the most wanted characters in significant numbers. You have largely refined the gameplay. You have done what many considered impossible. If you change gameplay too much, you alienate fans from the typically accessible Smash Bros. series. If you cut certain characters, you may significantly cut down on how many fanbases are being included, which can also alienate fans from the big wacky wahoo fighting game.
2) I feel, and this has been said many times by many different people, that Smash fans have an echo chamber problem. It's largely a problem with the internet as a whole (Twitter, Facebook, etc) but when folks hole up in a particular area and are surrounded by nothing but their own viewpoints, narratives shift and toxicity...well, toxicity thrives in an area where challenging views is unacceptable.
There were many characters who, I felt, had been echo chambered into relevancy. Geno was one. No disrespect, but Reimu was absolutely another one. When you get enough people thinking an idea is going to happen, and more people believe in it, the more accepted it becomes. To me, that was exactly what happened with several top picks, and it's not just Ultimate's hype cycle. For Wii U, Ridley and K.Rool fans suffered this fate, and Crash/Dante stans believed a little too hard that their characters were in. When things aren't often challenged,
3) I really think folks need to slow down, just a tad, with looking toward Smash 6.
Like...I guess I'm an old man (at the ripe age of 28) but this roster is just...incredible. Decades of series represented, probably thousands of years of gaming history (combined, not chronologically) fit together, tons of dream matchups (Mario vs. Sonic, Link vs. Cloud, Marth vs. Link vs. Sora vs. Cloud vs. Shulk if you wanna go into gaming swords too) amazing music...and the best part is that there is so much polish to it, that I personally believe it's taken for granted.
Like, once again I mean no disrespect here...but the polish of Smash compared to, say, Nickelodeon All-Star Brawl is something that can't be understated. It's on two entirely different planes of existence. Yes, one is made with a comparatively astronomical budget to the other, but the characters look, sound, and in some ways feel accurate to play.
Really, I feel the only thing Ultimate could have done better was authentic rivalry/dream matchup sequences. Like, something for Mario and Bowser, or Link and Ganondorf, Cloud and Sephiroth, that sort of thing. Character interaction would really go a long way. But there are plenty of things to do in-game, and it's amazing enough to see these characters together on the screen that it can be easy to forget that Smash started with just...12 characters, 4 of which were unlockable.