Smash is way too popular for Nintendo to stop making, at worst they'll just port Ultimate to future consoles and maybe add a little bit of additional content each time.
That's what really I'm worried about, I guess. All I can say is, if someone else makes the next game, it better not stray from the usual format.
And these big companies really only care about the money. Frankly, I wouldn't mind if they just kept adding DLC to Ultimate.