Yes, and most do. Many will semi-routinely increase the number of confirmations during times where it could be necessary (like chain splits or network upgrade e

Yes, and most do. Many will semi-routinely increase the number of confirmations during times where it could be necessary (like chain splits or network upgrade events), or adjust the number of confirmations depending on the size of the currency and overall usage. Another mitigation is to shut down trading until the attack is over. Maintaining a 51% attack is expensive (assuming there is at least SOME usage of the coin), and if you just say "we won't be doing business for the next 24 hours", the attacker now has to maintain that attack for the next 24 hours, and if the company wants, they can just extend the time. Eventually (hopefully!) the attacker will run out of money, and the "correct" chain will take it over again.