If ETH can still be this profitable after all those millions of Asic miners around the world then its possible that other PoW coins will continue from where ETH left off, its only a matter of time, once every coins are worthless to mine the difficulty will start reducing, this is where I will get in.
Of course you are correct. Mine while diff is low on a variety of coins where mining them is cheaper than if you converted the most profitable coin to that coin. But this process can take months or years.