Let miners leave. Let difficulty fall. Others will step up and fill that gap.
I think you might underestimate the amount of sha-256 horse-power that might be leveraged against the network once mining becomes unprofitable.
If mining is no longer profitable, mining hardware will be sold at clearance prices.
It all contributes to diminishing the security of the immutable ledger.
Could you eleborate on that scenario?
At a certain point in time, a group of mining pools decides they earn too little for letting their computers run.
So what option do they have?
1. Shutdown and leave. Like others did when the price fell below 300.
2. They introduce a new 'fork' which gives them more money and turn their hashing powers towards that fork.
When this fork happens my guess is that it will be so obvious to the whole world that it's just to enrich the miners, it won't happen.
How? Maybe by that time everyone has a small '21' asics in their home powered up. Governments who don't want to lose money will create laws against hijacking earth's currency. I'm just thinking out loud, but I'm confident a solution will arise.
// about the laws thing; ideally bitcoin becomes resilient that it's not necessary.
Sadly I share this world with people who like to abuse everything and anything. So yeah, we probably need laws against those people.
"Could you eleborate on that scenario?"
Sure.
"At a certain point in time, a group of mining pools decides they earn too little for letting their computers run.
So what option do they have? "
As a profit-mined miner myself, I would expect that the best option is to sell non-profitable hardware to the highest bidder.
Once the hardware is sold, the new owners of said hardware can mine whatever blocks they want. They can include or exclude any transactions as they see fit.
Are you beginning to see the problem?
"So yeah, we probably need laws against those people."
"laws" that can't be enforced are useless. If you are expecting to rely on force of government and enforcement of "laws" , you're gonna have a bad time...