they can open a 51 attack any time they want.
I would agree with you if 'they' was a single entity, rather than the most populous country on the face of the earth -- or even a half-dozen
decision makers running a half-dozen
pools, which each aggregate the hashpower of
multitudinous hashers, all in competition with each other over solving the next block.
But as it stands, there is no singular 'they', so no 51% attack.
i see they might change their mind and work together if something bad happen to because, sort of "this ship is dead let's act criminal now we don't care anymore"
but how the thing are going now, it's not so unbelievable to think that in the future only few of those pool will mine bitcoin, so the thing will certainly get worse from now