The Bitcoin network adjusts its difficulty based on the amount of computing power available so that the discover rate is relatively constant, around every ten minutes.

Adding more compute power without an increase in regular currency value per Bitcoin will just make it less likely to break even.

Not saying it couldn’t happen, but the odds of it making enough money at scale and without asic mining gear seems unlikely.

I don’t know about other cryptocurrencies though - they might have different designs that are more amenable to clandestine shenanigans.