I could see this genuinely being Sakurai's last Smash game; the title implies finality, and he went through the effort of bringing back every character and (almost) every stage to make the "ultimate" Smash Bros. game, a celebration of its long history.

That being said, there's no way in hell it's the last Smash game, period. Even if Sakurai steps down, Nintendo's not going to let one of its biggest franchises die off. Personally, I'd want Shinya Kumazaki to helm the games in the future; he also took the Kirby franchise from Sakurai, and his style, experience, and attention to detail are top-notch for this kind of series.

The smash games are secretly his hocrux's and he's spreading his soul out so thin its starting to affect his physical body