Look at the hash rate on Ethereum, 442 Th/s, Conflux has 1.42 Th/s, Ravencoin has 9.77 Th/s, ETC has 7.6 Th/s. Add up every other GPU minable coin's hash rate and it's a drop in the bucket to Ethereum's. 8 Billion dollars a year (just in block rewards let alone transactions) is simply too much for the market to absorb. Miners will start selling their rigs like crazy until equilibrium is met and profitability is had. Yes, people with free power or close to free power will keep mining, but the vast majority of the market will sell off their rigs, especially once we enter another bear market and profits are reduced even further.
Regarding new projects, pretty much all devs are moving to PoS for countless reasons. Environmental, governance, etc. I agree that security is much harder to guarantee under PoS early on, but security against attack vectors hasn't been much of a priority for a lot of newer projects.