if enough regular contributors agree that the changes should be merged, then they will be merged.
By whom? The code doesn't magically merge itself. Which individuals are responsible for the merging?
the change was merged by someone else.
By whom? The last I heard (which admittedly was quite a while ago), Wladimir was the only one with merge access. Who else has merge access and when did Wladimir give it to them?
If you are missing any of those 3 things, then you can't get the block size increase in Core. At the moment, this is stalled at the convincing Wladimir J. van der Laan step. It isn't clear how many miners would adopt the version if Wladimir J. van der Laan approved it.
He's not the only person who is NACK'ing those proposals. Almost everybody that is part of the Bitcoin organization on Github (not all are committers) have been rejecting those proposals.
Certainly, however, those with merge access have the final say. They can merge something without approval, and while the other developers might scream and shout about it and demand that no users install it, the code will still be in that version of Core. They can refuse to merge something that has approval, and while the other developers might scream and shout about it and demand that it be merged, the code still won't be in that version of Core.