Bitcoin has various checks and balances that help keep everything honest.
Even if a pool had 60% of the hashing power, they couldn't reverse 6 blocks without anyone noticing that it had happened.
There are sites which monitor the blocks and estimate the percentage of the blocks found by each pool.
In a way, bitcoin doesn't depend on the majority of miners following the protocol, it depends on miners believing that a majority of the other miners will follow the protocol.
If a miner has 5% of the hashing power and believes that the other 95% will follow the protocol, then the system should be set up so that it is in that miner's interests to follow the protocol too.
This is why soft forks work. The formal process convinces all the miners that the new rules are locked in.
In a system where miners can vote to cancel coinbases, each pool has an incentive to vote to reject everyone else's blocks.
Pools on the receiving end will be less profitable and lose customers.
It is possible that "predatory" pools would lose hashing power as miners switch to other pools, in protest.
The proposal allows "established" pools to vote to disallow new entrants. They could even justify it by saying that those pools haven't invested in "anti-double spending" infrastructure.
The proposal doesn't suddenly give the majority the ability to do it, but it isn't clear that making the process less disruptive is a good thing.