Besides collusion is unknowable due to Sybil attacks.
Collusion to do what?
You can collude to attack, but that would be visible.
If you collude with a bunch of other miners to pick each other's noses, okay, maybe we can't see that, but I don't care. That isn't an attack.
Attacker only needs 51% of hashrate to censor transactions perpetually (and less % to delay transactions).
To censor, he has to orphan other miner's blocks that do include the transactions. That is visible.