That's fair, what if another PoW coin gets really popular?

The good news here may be two-fold

1) the existing group of public chains have matured greatly and their network effects are quite strong now. It is no longer so easy to start a new chain

2) virtually all new chains are PoS because it's understood to be significantly more secure and less expensive vs. PoW

3) public chains take years to grow, and right now, there is no PoW public chain on my radar that has a chance of generating nearly as much mining demand as Ethereum (and Bitcoin but ASICs)

For those reasons, the idea of a new public chain appearing from nowhere and impacting GPU demand seems in practice unlikely to occur.