According to some comments from the core Bitcoin devs if an attacker has 51% of the hash rate they can selectively deny other people's transactions but it takes a much higher percentage of hash to be able to rewrite the blockchain.
Complete opposite - A centralised system requires you to trust the single party operating it, but that's super transparent. Either you trust it or you don't, but it's clear what you're getting yourself into.
Meanwhile, blockchain networks advocate a functional network that doesn't require trust - it's kind of predicated on the fact that you _can't_ trust anyone. But if a single party obtains 51% power, you have to trust them anyway. If you're going to require trust, you may as well just do away with the whole complicated setup and be explicit about it.
51% attack is an issue that people have been concerned about for awhile. But an actor having 51% of the mining doesn't equal to poorly implemented centralized database. And a lot of ppl are keeping a look out for this issue so my guess is that even if one bad actor who happens to have 51%+ tries to hardfork, the rest of the community won't move.
There are efforts to deal with this issue. One example being the whole debate around moving from PoW to PoS and other systems.
This is the problem - these blockchain ideologists only tend to see problems and solutions from the perspective of the actual cryptocurrency blockchains. Real world is sadly a little different...
