If profit was consistently 0.05 then yes, but once MEV profits surpass 0.5 fee this behavior becomes profitable, e.g. when there’s a highly profitable frontrunning opportunity. MEV profits will fluctuate, and often stay below this threshold. The problem is, once players start engaging in this behavior (at a time when it makes sense because MEV > 0.5) all the proposers will want to defend themselves after getting 0.5 a couple of times. The simplest defense will be to work with a trusted pool of block builders. At that point even if MEV profits drop back to 0.05 and the collusion attack stops, the proposers already centralized the builders into trusted pools.
In other words, as soon as MEV profits cross the 0.5 threshold once, the network switches to a more centralized state and there’s no trigger to ever switch it back.
Am I missing something that would stop this from happening?
Right. No additional property is needed.