it'd get buried under everything else releasing this fall. Fallout 4 announced in early June. This is in late July. They should've shown it off at E3 if it was coming this fall.
I also think that they'll put it on NX, and a late port of that won't fly.
Sonic, at the very least, has a dedicated audience that will likely show up guaranteed if the game is any good.
And especially with how fans have been in light of how long it's been since the last major/mainstream release, the thirst is definitely real. So in that respect, I'm not that worried. It really just comes down to being what kind of expectations Sega will put forth with this particular game.