Firstly, I think you underestimate how quickly good faith actors with slightly different configs would come to agree. A handful of blocks should be enough. Secondly, if reorgs start becoming a problem, exchanges and merchants could monitor for a situation with two competing chain and temporarily suspend processing. There is still the possibility that some one will suddenly reveal a long chain they had kept secret, but anyone doing such a thing is very suspicious.

Please post your suggestion in the issue I linked.