[biafra]

The Bitcoin network cannot know or should it care about a fiat exchange rate. Which currency should it look at? Dollar? Euro? Why? If the block frequency of ten minutes is not met, the difficulty is automatically adjusted after 2016 blocks (ca. 2 weeks) up or down.

[ifdefdebug]

I think miners have to know and care about a fiat exchange rate, at least until electricity can be paid for in BTC. So that's where I thought a threshold for miners running their equipment without losing money might come from - and thus a never-come-back dropdown limit. I really don't know, just curious. But probably automatic adjustment takes care of this, as you said.