It will be very unprofitable while mining BTC will be much more profitable due to the drop in hashrate on the BTC chain
There is a catch here. Bitcoin minting reward could be claimed after a few dosens of blocks IIRC. If 80% of hashrate goes to b2x chain then bcore slows down 5 times - before the difficulty recalc. So the miner choose between claim 1500 in weeks or 7500 in months.

Also there are periods of time when large miners minted at loss compared to bitcoin reward. BCash miners did that.