Ok, it seems there is a bit of confusion here. Despite the fact that Sonic Team is "seperating" from Sega, they are still PART of Sega. Here it comes straight from the Official Dreamcast Magazine.
In a move to make its first-party titles even stronger, Sega of Japan's internal Research and Development teams has been redefined as financially independant, seperate "companies" within Sega, and given a carte blanche to name themselves whatever they want.
As you can see, just because they are "seperated" from Sega, they are still loyal to their parent company. Also- it was not Mr. Naka, not Sonic Team, but Sega itself that decided to let the R&D teams become more independant. Yes, there is a small chance these 'companies' might develop for other companies, but that is very slim, especially for Sonic Team. Sonic Team's home, it's parent, is Sega. If they were to stop developing for Sega, yes, that might bring bad news. But they're not stopping.
Really, just don't jump to conclusions! These R&D teams are technically still part of Sega itself- there is almost no way they would take major risks in working for other companies. And as for Sonic Team, who SAID that they would stop making Dreamcast Sonic games? It's still the same old Sega, folks, read: They are not leaving Sega!