After todays merge, wouldnt Eth miners who were using cheap electricity to mine, switch to Bitcoin/BCH mining and thus cause a difficulty increase, raising the cost to mine new coins? It could cause a small price bump for these.

The GPU miners will move on to other coins and realise they are no longer as profitable to mine as Ethereum due to increase in difficulty forcing most of them to sell off their GPUs and cease mining anyways. Btw ETH had more than 90% of gpu mining hashrate before the merge. No way a significant portion of those 90% can just mine the other coins profitably unless the other coins grow to the size of the network effect Ethereum has.

credit u/tatabusa

Edit: if they switch to new mining SHA256 hardware and use their old facilties and electricity to now mine to BCH, lets say it had cost $100 to mine a BCH, so miners dont mind dumping for $100 worst case. But with new miners if the cost to mine rises to say $120, then miners will be hesitant to sell for less than this price. Causing a price increase.