There isn't a single owner of the bitcoin repository actually, if you go to Github you can see it's actually a Github Organization with most of the active contributors inside. For any PRs that are made affecting the bitcoin client or daemon software, docs etc. these people are the ones who have to come to an agreement on the change.
If the change however is related to the bitcoin network, such as a BIP that is being made or revised/replaced (there's a nice overview of that process
here which I will summarize), the bitcoin contributors are only the first line of determining whether a BIP, which is (paraphrasing) submitted to the mailing list first, will be rejected either because it's infeasible or too vague or else move on to the next stage. And that next stage is the amount of acceptance it gets from the community as the result of the BIP being submitted in a pull request. This stage is where e.g. bitcointalkers and r/bitcoin folks can raise their objections and even block the BIP from proceeding. Then the last stage is deployment which depends on the number of node owners and/or miners that decide to implement this BIP by updating their clients and software. Bitcoin Core developers only have full control of the first stage not the other two.
After the pull request has been accepted, I think miners will also need to update their software accordingly. Right?
Miners are the ones who actually decide what the longest chain will be.
If they do not update, new blocks in the longest chain will be following old consensus rules.
