this thread is about bitcoins. So I thought its implied we are talking about bitcoin. Maintaining a POW block chain is inherently wasteful, but yes POS blockchains can maybe save this.

Even with bitcoins just consider what happens once all bitcoins are mined. Does the whole thing collapse? Or do bitcoins continue to be used similar to POS?

Then the transaction fees associated with every transaction fund the miners.

The block mining algorithm doesn't change, right? So I don't see how it will move to POS. That will require some sort of buy in from the users. Every time something new is introduced in Bitcoin the legacy remains and the new ideas fork the chain into their own thing. Usually these alternatives either die or ignored.