A potential attacker would need to act economically irrationally to successfully launch a 51% attack, when only looking at the attack in terms of bitcoin.
That "economically irrational attacker" would be wasting resources. The network is made of people, capable of coming to consensus if there was a direct threat.
What the troll said was that the miners could collude. But would they? They would be opening a strong debate for a POW change.
As PrimeNumber7 pointed out, they would be economically irrational
with regard to Bitcoin's incentive system.
That doesn't rule out incentives external to Bitcoin. Let's say the Chinese government simply wanted to crush Bitcoin with a sustained 51% attack. They could systematically co-opt mining farms, threaten pool admins with imprisonment if they don't cooperate, etc.
Not that I think it would be successful long term -- I'm just pointing out a situation where a majority of miners might be incentivized to collude against the network. Indeed a POW change could eventually thwart the attack, but it's interesting to ponder what kind of damage they could do in terms of market confidence and adoption.