I think there will be a healthy amount on both sides regarding whether the fans will buy Shenmue III if Yu is involved or not. Though even if every single Shenmue fan jumped on board and bought it, I don't think those sales would justify them making another if III didn't finish the job. They'd have to do something with the first two in order to get the third made, at this point.
Regarding interest in the IP, it is my opinion that the company has every intent to make the game, but don't want to take the possible risk and absorb what might turn out to be a colossal failure. Shenmue is huge, and on the off chance that sales are absolutely dismal, the series will never see the light of day ever again. However, if they could license out the IP and just take a check of the some of the profits without having to fork over much money, I believe they'd be very interested. What company wouldn't? If YS Net had the capabilities (some day) to make it themselves or team up with another development team, then I don't think SEGA would stop them. That being said, I don't think they want to contribute much, be it manpower or financial support, to a game that is considered to be so risky. Now with THAT said, I also think SEGA should realize that Shenmue's circumstances were very odd and unfair to the game and the team itself...the shelved Saturn version, then the US cancellation of the Dreamcast, development issues with Shenmue Online...really the only solid data we have to go on is the success that was Shenmue I, and the failure that was Shenmue Gai. Shenmue II and Shenmue Online are both multifaceted, complex, issues.
Just my two cents.