I would like to know what is happening inside MS and what is actually behind this move. From the outside looking in we can speculate all kinds of abusive behaviour, especially with Microsoft's past in mind. But there is so little information...

Say there was some QA team that found that some serious protected fallback mechanism was required to reduce support costs, that might be somewhat valid. But even then you'd expect to at least see some information appear about that, and that hasn't happened either. Heck, even security-wise they could come up with a story on how browser hijacking and protocol handler hijacking in windows is super bad and this was their only move (not true of course, plenty of other moves available) to mitigate it. But we get nothing, except broken behaviour and no real publication as to why. They can't throw it all on UX either, because if that's what they wanted they wouldn't be able to support an incompatible variety of PCs.