Some is used to pay for electricity and gear, the rest is kept in the coins they support. Some where mining ethereum when the price was $1 a coin, you don't need many coins to reach millions of $$$ with todays price.
Some of the biggest Ethereum owners are old miners. I read that 7.4 Million Ether, Close to $30 Billion Locked in ethereum 2.0.. When the oldtimers start the sellof down to $40. the 80billions locked will be reduced to 0.3 billion.
Not a difficulty bomb anymore, but a squeeze..
Well as soon as eth devs does the merger, all those staked coins are free to be sold in the exchanges, if the price is around 300 usd then I think some will sell cause everybody needs money, the majority will keep their coins, if eth at that time is high lets say 2k or higher then yeah, will be a mass selloff, making eth to crash to less than 300 usd and people dont buy prior bear market cause they know there will be no profit if they buy the dip, meaning, eth will crash very hard, how low? that is the question.