Because adding a 2mb blocksize in excange for SEGWIT would keep the chains & communities from splitting in two and destroying both sides. Even if it's not necessary by luke-jr's opinion - if it is 2MB blocks or 2 chains which one is really worse for bitcoin?
This is the third time around. BitcoinXT, Classic & now Unlimited. I fully expect this one to die out due to Unlimited's lack of professionalism & experience. Then continued gridlock on SEGWIT. At some point the industry will decide it's tired of waiting for > 7tps
What you are saying is that core should agree to increase the blocksize limit to 2mb merely because if they do not, then the BU miner group is going to fork. That is hardly a justification.
The burden to show why it is needed is on anyone proposing, and then it gets discussed and if core agrees then code gets written and tested and then signaling and implemented.
Core is not any monolith or any small group. You gotta convince a mostly decentralized... if you cannot get the stuff through official channels, then bitcoin just stays the same. Bitcoin is not broken, so staying the same is not the end of the world - even if some group ends up forking into some alt that might initially have more hash power. Once they fork, then it is much more likely that 95% segwit consensus would be achieved. thereafter, bitcoin begins running with seg wit (whether it has smaller hashpower or not, bitcoin will be fine).
In other words, merely the threat of a fork seems way too insufficient in order to cave in to some proposed change that is not technically justifiable.