I wouldn't consider Brian May to be a 'transcient celeb'. He did co-author 'BANG' with Sir P and Chris, he did return to complete his Phd, he was a long-time friend of Sir P, has been actively involved in promoting astronomy, and is worth about £90 million so I don't think payment would be much of an issue (!). He is alspo very committed and active in several 'causes'
Yep my only concern would be him being committed to doing it as a permanent presenter with all the other things he has on.
A lot might depend on how the BBC see Chris, Pete etc as they basically carried the show for the last few years, it's possible for example if Brian May took it over they may want to get rid of Chris and Pete as permanent contributors, that might mean Brian May doing more of the reporting and travelling which he may not be able to commit to.
Of course it's all speculation until we hear something for certain, certainly the Feb show is going out with Chris, Pete and Lucie.