Before Ethereum there was Litecoin which was a GPU mineable coin at that time,precisely in 2015.This is to show that always in history there has been a top PoW coin leading and then every other type of coins followed,at least this is how I read it.
Anyway so far PoW coins have always been the better ones with a much larger market share as long as I know it is still Bitcoin and Ethereum the two big ones which have the greatest market share compared to almost all other coins,I doubt this will change any time soon,let alone if it happens because of a PoS coin.
I started in 2014. At that time Litecoin ASICs were becoming reality, Darkcoin was moving from CPU to GPU, and Eth didn't exist yet.
Nothing really compares to ETH, every other major GPU coin prior was taken over by ASICs. Eth has been going strong with GPUs
for much longer than anything else.