> The more people collude, the more profit they get, the more rational it is to join them.

This is not true. Colluders diminish the value of their PoW reward by reducing the trust of all other participants in the system and honest actors have an even strong incentive not to join them. Just a few years ago a mining pool that gained a very large share of the Bitcoin hashrate had members choose to leave -- https://www.coindesk.com/bitcoin-miners-ditch-ghash-io-pool-...