A more "realistic", but still not going to happen, scenario is that a small group of miners and nodes get together and cause a branch. Let's say 5%. They would have to make changes to deal with the 95% drop in hash rate on their chain plus whatever other "fixes" they want to implement. The main chain would experience a 5% drop in hash rate and would probably not even notice the difference.
The remaining 95% could simply wait until there was a way to sell the new alt coin and then dump all their free alt coins on the poor alt coin market.
Add to that the 95% miners would have a very vested interest in ensuring this imposter dies in the cradle. This is even more likely if the imposter tries to use the bitcoin brand because it creates user confusion and negative PR. So say 6% of those miners could temporarily take a small paycut and 51% attack the imposter. Exchanges accepting the imposter coins could potentially go bankrupt, merchant adoption would essentially be zero. In all these scenarios what the person proposing it usually fails to consider is the application of risk.
It is interesting to note that as the last subsidy cut approached some predicted the same thing happening then. Of course it didn't; miners tend to be pragmatic. 1 bitcoin in the wallet is worth more than a scheme to have two.