I'm not an expert, but if 51% of the existing hashpower doesn't want a change, it sounds less a 51% attack and more like miners voting against something that isn't in their interest. The whole point of blockchains is that the incentives are supposed to be aligned between miners and users. If that isn't the case here, it sounds like a problem.
You should think of miners as bodyguards. They aren't intended to be active participants on the network, they are intended to provide the service of securing the network and serving the interests of the users of the network. If a group of bodyguards at a concert vote democratically and 51% of them decide the singer shouldn't be allowed to go on stage and sing (because of a grudge or whatever), is that a problem? Yes…
Miners are not supposed to serve anyone but themselves. Everyone for themselves. A right system economically incentivize the right action by design, like in Bitcoin. Important to understand the first principles before judging any distributed system.