What exactly can he do if he somehow gets to own >50% of the hashing power through his pool?
I don't think that is a possible scenario, bitmain owns two of the largest pools and they are from that 50%, they also manufacture most of the gears and do own a ton of them, in a nutshell 50% of hash power is way above binance's capabilities.
Keep in mind that miners can switch pools in a second, also it is for Binance's best interest to not try and do anything obviously damaging to bitcoin.
Yes well let's not forget they did ask for a "chain roll back" after the hack so if they start to control the hashrate then it could lead to something bad in the future.
IIRC CZ didn't ask for a roll back, he talked about the pros and cons of it and i think he mentioned that roll backs are bad for bitcoin and he wouldn't want to see them happen, some digging into his Twitter needs to be done to confirm this.
I am with you in regards to having more "major" pools, if it was only a mining pool they are up to i would support that, but the CMC thing is what makes me worried about binance shady goals, there is also the incident with DGB where binance asked how much can dgb team afford to pay to be listed, all of those things make me not trust binance for anything outside of thier exchange which they have proved to be good at running it so far.