While I don't think Smash will "end", I think that there could be changes going forward that result in Smash "as we currently know it" to end.
I mean, think about it. While there's hardly any way for Smash to sell poorly, they've gone all in on making the mascot crossover the best it can be in the most effective marketing method possible: having an all-star roster of characters. We're at a point where with basically any character being cut, that character would become the among the highest requested characters.
We're also at a point where few of the remaining non-playable requests don't have nearly the same fan support or demand as previously large demands, and their role in their home games isn't necessarily large. For "new" series, there are few that have regular prominence in promo material and releases. Ultimate covers many bases.
Even if you add Dixie Kong, Bandanna Waddle Dee, and Waluigi, that's three newcomers. If Golden Sun and Advance Wars aren't revived, those two series are hard on luck to get a fighter. Sure, you'll get the newest Pokemon and probably Fire Emblem lord, but where else do you go? For another example, Chibi-Robo is dead. New character additions would probably be overshadowed by veteran announcements overall, but there isn't a major pool for 1st party newcomers like there once was.
This would result in, most likely, changes to gameplay and game modes, which isn't as much of a draw as characters are currently. It's not a black and white situation at all. While Smash will likely continue, the question is not necessarily "will they" but "how will they". It cannot be denied that it will be extremely difficult to outperform Ultimate in terms of scope and audience. I'm not entirely sure how this will happen, if i'm being honest.
The fact that this crossover may never happen again is a reason why I'd like to see it get even bigger. It'll probably never happen again, so I say go all in.
i mean, i don't really agree with the notion that first-party options would be limited for a smash 6, simply because it's not like we're getting smash 6 right after this one
if we go with the assumption that we won't get a new smash for a
while, then by that time there should be a lot of new characters and series rising to prominence (and probably even a few older ones coming back). the current options we have? sure, i can see that, even though there's still a crapton of first parties i'd love to see, i have to agree that the vast majority of them don't really have star power.
to better illustrate my point, let's look at the hypothetical scenario where it takes 10 years until a new smash. the longest time period i've seen thrown around until a potential smash 6. 10 years was also the period of time between brawl and ultimate. of course, during that time period the at-the-time current nintendo landscape changed heavily, so if we take a look at various additions in smash 4 and ultimate, we can see that between brawl and ultimate, the following things happened that affected the two games after brawl:
-punch-out and kid icarus were revived
-splatoon became a thing
-xenoblade became a thing
-isabelle became the de facto mascot of animal crossing
-fire emblem became a major ip
-arms became a thing
and that's just the ones that affected smash
as this demonstrates, once smash 6 comes around, there will probably be a lot more viable first party choices again, provided we don't already get it 3 or 4 years from now (and even then, it's not like we HAVE to have recent first parties only). assuming it'll be a while, we're bound to get a bunch more new characters, series and revivals until then.
