Apart from the fact that I don't see why Macca can't just buy them if he cares so much (I'm sure he's still got £120 million available), I'm also not into ideas like turning it into a museum or getting it listed.
For one thing, it's a wholly unspectacular building, so it doesn't deserve any listing, but also the thing that matters most about Abbey Road is not the building - it's the zebra crossing.
The fact is that you don't need studios in nice, expensive places like St Johns Wood. Probably made sense when you needed very skillful people to record onto master discs, and when people were less mobile. But today it can all be done digitally, and it's unnecessary.
So, let it get turned into flats or a hotel... far more useful for London.