Indeed. I'm not a zealot either when it comes to classic Sonic's design. In fact, I think modern Sonic looks pretty damn cool sometimes, like in Unleashed. But if asked which I'd prefer, I'd no doubt choose classic design over modern. It's an aesthetic choice that's by no means a deal breaker either way, but it can be nice little superficial touch for Sega to consider. Pure fan-service wouldn't hurt.
Nintendo apparently sees the value of having different Marios for different occasions, all co-existing in the same marketplace. Each design fits the given project perfectly:
Sega will likely never take the Nintendo route though, probably out of fear they'd mess up the good formula they got going (financially). But I think in this case, diversifying their icon's image (for specific projects and audiences) might actually
strengthen the brand as a whole.