How would they control the code if Core devs would follow the 2MB increase?
We're far away from not being able to scale on chain and increasing the blocksize doesn't mean there can't be developed on other scaling solutions.
If other solutions works and get adopted then the blocksize keeps smaller or gets smaller again.

But its simple. If majority mines the segwit2x chain then core gave the main coding away.
Nothing stops them from following that chain.