When Sonic debuted, he was presented as a hip, sassy rodent with an attitude (in contrast to kid-safe Mario). Everyone cool had attitudes in the late 80s and early 90s, so Sonic needed one too. Sonic 2 comes out, and he's still a totally awesome guy. Except now he has Tails, who is sooooo kawaii! Then in Sonic 3, he's snowboarding and Knuckles enters the picture as a streetwise, mad phat monotreme with dreadlocks.
The Saturn Sonic game was going to be named "Sonic X-Treme".
Then the Adventure games came along, and they feature soundtracks with rapping, a new, grafitti wildstyle-inspired design for all the characters, and product placement in the form of grinding shoes.
So maybe Sega is pandering to what's trendy by creating a black, angsty hedgehog and giving him a gun, but haven't they been doing that all along?