Why should it? You're saying as if Smash won't have legs during its LT or something. Smash will be fine, whether it be 12-20 million sold. That's more than any fighting game could ever dream of, let alone having the kind of roster Smash Bros. Ultimate will have.
I just saw a comment here that said that it's laughable to think that Spiderman or God of War will sell more than Smash, and that Smash will sell more than them combined. I made that comment because that Smash>Spiderman+GoW was quite amusing.
"The rumours of my death have been greatly exaggerated."
- Single-player Game