In all due honesty, what good would mobility do for Bair? I guess if Sonic had Wario's acceleration, he could do zig-zags with his Bair, and perhaps it harder to guess where Sonic will land assuming he does it on top of a shield, but if Sonic could cover more distance in less time (air speed I think?), then he'd be further left or further right after the Bair connects, would it not?
Unless I have my terminology mixed up again...
Anyway, as for the discussion, what can I say that hasn't already been said? The only problem with Bair is that it's so good that it ends up being used in every situation most other attacks can do, outperforms it there, and weakens the Bair in the other utilities it possesses. Use it as a kill move, and you find yourself having a more difficult time racking up the damage. Vice versa, and you find yourself with the opponent living longer per stock.
Now it isn't necessary a problem if Sonic can't kill his opponent, just as long as Sonic himself doesn't die beforehand. so perhaps Bair shines in that it's safe, and does 13% if fresh, and its sex-kick hitbox still does a hefty amount of damage, and leads into follow-ups.
Let's pretend that Bair is used for the kill. Could somebody enlighten me how a Sonic plays in the time that plays out from 0% to kill percents? I honestly don't have a clue, I don't think that is even realistically possible. Grabs and weaving in can only get so far before the opponent will figure out that Sonic doesn't have the range or safety in most, if not all of his other attacks to match Bair, and then plays excessively defensively to keep him out and cause problems.