If "one guy with commit access to the bitcoin core repo" pushed a change that wasn't in the interest of miners and heavily invested groups, why would they install it?
Foundry + AntPool alone make more than 51.7% of the hashrate. Next are F2Pool, ViaBTC and Binance with about 10% each.
So this "decentralization" requires what, the agreement of 2-5 parties.
Realistically, if the pools wanted, BTC core could be replaced because the software is mature and the big pools can trivially hire somebody to work on it if they had a need to.
The big exchanges probably also have some say.
But guess who doesn't? Anyone else. People running their "full node", or just random joes using the system don't really have a vote.
It's a very much top-down system with a few fat cats sitting on the top, and none of them were even elected, and quite a few are effectively anonymous.
As long as some suckers are trading actually liquid assets for the instruments on the sabotaged chain, it is economically rational for the miners to mindlessly rubberstamp the rules of the chain.
Which is exactly what we have.
Former Bitcoin Cash developer Amaury decided he wanted a subsidy to himself. He forked, forming his own extremely minority coin, Bitcoin ABC (now eCash) which pays him 8% of everything mined. Despite being very conclusively rejected, and the chain having undergone attacks, eCash still retains some value, and Amaury still gets to pocket a percentage of everything that gets mined.
The underlying code for that is the same as for BTC. So at any time, if the cabal thought cratering BTC would be worthwhile if they got to pocket enough of what remained, they could do it with complete impunity. All it'd take is the agreement of a very few people and a viable exit plan.
That's an interesting thing I didn't realize until not very long ago. If setting fire to 99% of the ecosystem allows you to pocket a fraction of the 1% that remains, and that works out well enough to not have to work again -- that's a very favorable tradeoff for a lot of people.