Preventing transaction fee collusion between large miners What would prevent the largest Bitcoin mining pools (top 3-4 have with 61-73% hash rate today according to link below) from colluding to demand much higher transaction fees to confirm transactions?

https://blockchain.info/pools

My question is somewhat theoretical and I am aware that many smaller pools would naturally oppose this. However even if transactions get confirmed by smaller pools eventually, when the majority of the hashing power is colluding to only accept high fee transactions, then the delay for confirmations of low fee transactions will be very significant.

Some countries have laws that may prevent this type of collusion, so please assume for the purposes of this question that the colluding parties are either in jurisdictions that do not regulate this or the pools operators cannot be identified.
In theory, preventing this problem is why miners exist in the first place. The sort of collusion you're describing can more generally be described as censorship: miners jointly deciding to delay certain transaction unreasonably long (up to indefinitely).

Ideally - and I'm well aware that reality is quite different now - the mining ecosystem is reasonably easy to enter for anyone, including people who feel they are being censored.

This is the difference - and the only difference - between using Proof-of-Work as a consensus mechanism and using a traditional (centralized) distributed consensus mechanism: anyone can enter, without asking anyone for permission (even anonymously). If we were fine with the set of existing mining pools to never change, we could replace it with a permissioned system.