So, guesses as to how much difficulty will fall after halving? It's going to be half before half after, so probably every last ditch miner's already in the game now, and they'll switch off as soon as we get the last 12.5 bitcoin reward.
Some news I read somewhere says it's monsoon now so power's plentiful where it's hydro in Asia so that'll actually help make up for some, so maybe the slide happens a few more adjustments down the line?
This is my opinion on that:
People expected a crash of the hash rate like that in 2016 but it never happened. The price actually crashed over 30% shortly after the halving and the hash rate still stayed stable the whole time.
I think there is a big misconception about what the "average cost" to mine a BTC actually means to the industry. A strong majority will still be profitable at these prices (some of whom will add more hash power after the next adjustment) and some others are capitalized to ride out the market for a while on speculation. Just my opinion. I think it will be a non-event in terms of hash rate, not the massive drop some are expecting.
Price has risen much more than difficulty in the past few weeks, which tells me weaker miners are already shutting down. All in all, I expect it to be a wash, with no major crash in the hash rate and maybe even a modest increase, depending on what the market does.