Nothing much honestly.

Mining in Bitcoin (and the other cryptos) adapter to the amount of miners, and will still produce a similar amount of coins and work as normal.

Sure, there are extreme cases like if a very significant amount of miners would suddenly stop, then the blocks would come in at a much reduced rate until the algorithm adapts itself.

And if too many stops mining though, then the chain becomes too easy to attack, which would (err, should) destroy its value.

I did use the word “significant” in my original question.

The following reply to this question makes it sound pretty dire:

https://www.quora.com/What-if-everyone-stop-mining-Bitcoin

During the crash in 2017 mining decreased drastically, I want to say by like 70%? There were slow blocks for a while then difficulty adjusted.

In the case of ETH, difficulty adjusts every block. ~15s.

A lot of crypto isn't mined in the first place. For those that are mined, increasing mining changes nothing, decreasing also unless you decrease it so much as to be trivial to outmine 50% of the total.