They can hold the bitcoin network hostage, that's how they will benefit. They can impose that they be paid for whatever they are doing and we can't do anything about it but heed to their demands. Unless we all move to another coin and forget about the chain they are in. I guess, it has an unpredictable consequence which could lead to great chaos.
If someone does this they pretty much guarantee that the Bitcoin network will be forever worthless. No one's going to trust it again. No one's going to award it any value. The world will have to wait for a chain immune to this angle before people get interested again.
The only people who'd do this would be those who want to destroy it completely. At present Bitcoin's enemies with anything approaching this capability are people who want to control and co opt it, not kill it.