For the sake of argument, Sonic was always trying to be the "cool" Mario from day one. Just reading the old manuals in Sonic Mega Collection brings back the memories. He's a radical dude, bad enough to save us from Dr. Robotnik, don'tcha know?
That said, all of Sonic's cutesy anime friends probably don't help him out in terms of targeting older demographics. I guess that's the cost of having all this graphical and audio power: people expect some type of a storyline, and at that point you have to start deciding what audience you'll cater to.
Edit: of course, to answer the question of "why do adults still play Mario games and not as much Sonic games?", I'd say that the side-characters (and cheesy voice acting that accompanies them) certainly don't help, but the big problem is likely the quality.
Double-Edit: Read the article, and it's saddening. I don't think Sega's American ventures have really produced much in the way of quality games...probably even less than their concurrent Japanese products...but they do seem to be improving the company's financial outlook somewhat.