if hashrate leaves difficulty will adjust and show will keep going .. what's the issue here?
in a normal world there are 1 block every 10 minutes on average which makes 144 block each day. 144 * 12.5
BTC = 1,800
BTCimagine you have 10% of the hashrate and lets say each day you get 10% of 1800
BTC = 180
BTC = $486,000
(assuming BCH reached equilibrium and is mining 1 block every 10 min on average) now in our fucked up world you can one day decide to switch your 10% hashrate on bitcoin to bitcoin cash. with it you can mine a lot more because first of all 10% on BCH is a lot more (nearly 80% of total there maybe more but lets take 80%) and also because of this big hashrate rise more blocks will be generate in 1 day. lets say 200 blocks
200 * 12.5 BCH * 80% * $500 = $1,000,000
right now ViaBTC that has 5% of bitcoin hashrate has mined 25% of bitcoin cash's blocks.
of course things aren't as simple as this and i am making big assumptions about numbers. for example a miner can not just sell the coins as he gets them. coinbase transaction (that 12.5) need to stay about 100 blocks IIRC or so before becomes spendable.