Im sorry i did it unorganized.... But was trying to see how ssbu legs can be. BotW and MK8D show how crazy evergreen legs can be. That said if ssbu has even worse legs than mk8d it can still cross 25mil
No game is going to have legs like MK8 D, but Smash Bros Ultimate doesn't need to have legs like that. I think BotW is a better milestone. BotW still has very strong legs even though it released over two years ago. Smash Bros basically need legs like that or maybe just a little stronger. The main thing is that it needs to show that it is going to keep selling strong even years later.
Right now my gut is that Smash is going to keep selling like that. Right now when people get a Switch, the biggest chunk of people see Mario Kart as that "must own" game, because basically everyone likes Mario Kart. After that I think the people who prefer single player games see Zelda as the next "must own", while people who prefer local multiplayer see Smash Bros as "must own". And I think this mentality is going to keep up throughout the Switch's lifetime. Other games may be popular, but so far no other game will have legs like these 3. (Pokemon might end up being a 4th game like this though, we'll have to wait and see.)