GPU miners will move on to other coins that are profitable to mine or show potential and others will just sell their rigs and cash out their eth or stake what they have mined and hodl over the years!
There are no official specs for casper which will roll out PoS so any speculation on whether it is too late to get into mining or whether you will qualify for the "backbone" of ethereum are not worth discussing at this time.
To learn more I direct you here: https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQ
3.1) No way to predict prices based on rumors.
3.2) No, Ethereum has proven itself to be far bigger than what any single mining group can promote!
RE: Eth: PoS?