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?
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.