Last time I checked, the past few mainline
Mega Man games were positively received both critically and commercially, so I'd argue the Blue Bomber's future is uncertain rather than nonexistent.
Pac-Man is part of the roster mainly because of Bandai Namco's involvement. Even if you take that away, there's still his iconic status, which he gained solely because of the 1980 arcade classic rather than the lackluster 3D games he's appeared in over the years. On the other hand, the
Sonic franchise has become notorious for poor efforts such as
Sonic '06 and more recently,
Sonic Boom: Rise of Lyric -- the latter of which was released not far apart from both versions of
Smash 4 and was the
series' biggest financial bomb yet.
I'm afraid that Sonic's chances of returning aren't looking good. It's not impossible, though. And I do see where you're coming from.
I don't want to turn this into a long-winded debate, but rather give my two cents.
Edit: I somewhat enjoyed Smash Tour. It took some time to grow on me.