WHEN THAT HAPPENS though, profitability on ALL GPU-mineable altcoins is going to collapse as all of the ETH POW GPUs (some MILLIONS of them now) look for new homes.
Lol man, this wont happen. Since they will deploy their PoS in batches, starting at 99% PoW the hashrate will slowly decrease over time, meaning other coins wont get a sudden influx of hashrate. It will increase over time. Another thing is that the amount of hashpower a coin has to secure its blockchain is a fundation for the price. Higher hashrate = Higher price since its more secure.
The plan is 99/1 split, test the POS stuff, then move to full POS, per my understanding of posts from the devs about the planning.
There is no "gradual shift" planned like you are talking about.
Is the testnet even working yet?