I know there's a lot of people (including myself) who'd definitely agree with that estatement, but one has to wonder: how long can Nintendo really keep supporting such a big game like this? Guys can't do the same amount of fighters in a year as they did last season, I doubt next characters could take as much effort as Steve did, and more importantly: the Switch won't be supported forever. There's that old phrase "what goes up, must come down" and when you reach the levels of quality Ultimate has, there's really no place you can go that isn't down (not necessarily in a bad way as next entries could still be solid games on their own right). Only other choice around is just to port Smash Ultimate each and every generation going forward, but that'd be wishful thinking.
It's difficult to say really. Nintendo's very much the kind of company that has historically considered released games "done" as opposed to companies like Valve who took to the live service model very early. In order to continue supporting Ultimate post FP2 Nintendo / Bamco would practically need to have a dev team commit to 1 or 2 years worth of additional dev time. I don't think Nintendo's structurally ready for that, even if they are inching towards this by their long term support of Splatoon.
It could be hypotheoretically possible for Nintendo to decide to do an Ultimate DX and then go live service with it, although there are numerous potential pitfalls. Mind you, I'm used to the live service model (having a lot of experience with Valve's Dota 2, which is very much in this category):
There's a reason why I continually bring up IceFrog, Dota's head developer since
2005, as a compare / contrast to Sakurai. Unlike Sakurai, IceFrog's so anonymous and so solely devoted to Dota that people seriously couldn't confirm his identity
until a court filing in 2017 did so. He's an extreme example, sure... but then IceFrog's not alone: the court filing notes a certain Guinsoo (Steve Feak) - ex-Dota dev that worked on LoL up until 2016-ish - Feak's worked within the same genre for a long time. (Edited -
3BitSaurus, I'll have to ask, do you know if Feak still works at Riot?)
And it's that kind of developer that live service model's often built around - those who have an absolute burning passion for their genre. Valve, Riot, etc. also have entire teams dedicated to their respective games, and those dev teams are generally not shrunken down in size either until long into a game's life.
Unlike IceFrog or Guinsoo, I get the impression Sakurai would like to take a break soon and then decide whether he should focus on a Ultimate DX or another project entirely. He's very likely not the kind of developer who'd readily approve the live service model either for entirely justifiable reasons.
It's hypotheoretically possible to port Smash Ultimate continously (technically Counter-Strike, a game that's been continually been played since 1999 and 1.6, is still played today even if it's changed a lot since then) although that might be too much work for Nintendo too.
If a Smash game is to go live service, I honestly predict it'll be a future game like Smash 6 or 7 that does so. One possibility then is that Smash starts small in "Beta" (like Dota 2 started in "Beta" and stayed that way for 3 years. Yes it was ridicolous), continually gain or regain new characters, and might even pass Ultimate's scale given enough time...
...But ooh boy I don't think the Smash fanbase at large would take kindly to the idea of a 3 year Beta.