ETH has a net hash of roughly 100TH which would equal 3.3 Mio GPUs mining at 30 MH/s (RX 470/480 or RX 570/580).
I suppose that most of these GPUS are in the team red camp. If ETH goes to PoS these GPUs will switch to something else but all Equihash coins are basically a bad choice.. high consumption and rubbish hash rate. Cryptonight would be another viable algorithm for them.

It would also be interesting to know how many of these GPUs are commercial/farms and how many are operated by individuals. Both have different thresholds on when to stop mining.

Over the course of the next 6 months I expect that everything stays relatively stable (similar to the period August-November)