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.