The name matters for certain franchises. People might buy Sonic for Sonic, but still think it's weird to see say, Activision, on the title screen. Shenmue and Powerstone still have a fanbase asking for sequels, so they'd be alright especially with good developers. Games like NiGHTS and Jet Set Radio without the SEGA name are DOA, though.
The thing is, no company is gonna buy more than one IP from SEGA without needing more people, preferably experienced with the franchise. And SEGA wouldn't do well trying to sell them one at a time. They wouldn't do Sonic first, and that's the hottest one. The prices would just get driven down and EA would buy Crazy Taxi for like 99cents.