Mining pools can't change the rules of Bitcoin. They can only change the rules in their own clients, which would have them producing blocks which are not accepted by the larger Bitcoin network.
Changes to Bitcoin rules require consensus by all the participants (people running and using nodes). The only things a majority of miners can do are attempt double-spends of their own transactions, or temporary denial-of-service by excluding transactions from the chain of blocks they mine. Any other changes will fork them off the network, and then they'd be minting coins with no economy to sell them into.
Ideally we would be better with increased decentralization. It could be a problem if they colluded and attempted to censor or double spend. Chances are they won't, because it will probably negatively affect their own bottom lines (reduced confidence in Bitcoin leading to reduced value of their own assets).
Secondly, any blatant attack on the network could be countered by the participants forking in a change in the algorithm used to do the proof-of-work. This would bankrupt the attacker because they would be sat on enormous amounts of now useless mining hardware.
Changes to Bitcoin rules require consensus by all the participants (people running and using nodes). The only things a majority of miners can do are attempt double-spends of their own transactions, or temporary denial-of-service by excluding transactions from the chain of blocks they mine. Any other changes will fork them off the network, and then they'd be minting coins with no economy to sell them into.