"If all miners do not make the switch to Proof of Stake, then there is the danger that ethereum’s blockchain might fork. A similar situation occurred in 2017, when bitcoin miners forced a fork in its blockchain by throwing their weight behind bitcoin cash. Ethereum’s founders, however, foresaw such an eventuality and programmed its blockchain to incorporate increases in difficulty levels for its mining algorithm"

https://www.investopedia.com/news/what-ethereums-difficulty-...