That's not true. While bitcoin mining is conducted through pools, what mining pools do is entirely public. Miners can easy choose another pool if one abuses their power.
For example, in 2014 a pool named GHash.io was close to obtaining 50% hashpower. It is customary for pools to stop accepting new signups when they are close to 50%, however GHash.io refused to do so. The miners organised an exodus and GHash lost half of their miners in a month.
I understand from what you're saying that bitcoin is continually dependent on social customs that have already reached the verges of failure.
At the moment (some) miners are motivated to avoid 51% attack because they gain more from protecting Bitcoin's value than they do from profiteering with a 51% attack.
If that stops being the case, even for a moment, then the 51% attack will happen.
For example, in 2014 a pool named GHash.io was close to obtaining 50% hashpower. It is customary for pools to stop accepting new signups when they are close to 50%, however GHash.io refused to do so. The miners organised an exodus and GHash lost half of their miners in a month.