> simply transition to proof
As opposed to Proof-of-Work, Proof-of-Stake is not as simple as it sounds. Any implementation faces a myriad of design challenges and potential attacks. Things like "nothing-at-stake", "costless simulation", "stake grinding", and "long-range attacks" [1]. And in the end, there is no objective truth about the state of the chain, as there is with PoW's simple longest chain rule.
There is nothing simple about a Bitcoin "transition" either, as should be evident from the endless blocksize debates, and resulting Bitcoin Cash chain splits. That was only about changing a single parameter. Now you're talking about changing the very essence of Bitcoin, with dozens if not hundreds more dimensions of choices and opportunities for disagreements.
[1] https://download.wpsoftware.net/bitcoin/alts.pdf