If you stop mining bitcoins, anybody with a small rig can produce a double spend and destroy all trust in the network. It wouldn't disappear, but the coins wouldn't be worth much.
Changing the block size was so controversial that it split the community and created years of bad blood and claims of shilling. I don't believe that the core devs are capable of making such a huge change as moving to PoS.
Bitcoin is whatever the majority decides... They could also increase the supply if they wanted to, or whatever other change comes to mind? Think of it as Bitcoin 3.0, 4.0, etc...