It sucks, but yes they should and they will if they have to.
Khabib does not run the UFC. WME and the UFC are a million times richer than Khabib will ever be. They're not going to let him dictate how they run their own business lol.
They've made it clear they want the rematch. Khabib can refuse of course, and he'll be stripped, and the UFC will move on with Conor vs someone else for the belt. The hardcore fans will call it bullshit (which it would be), but the casuals will move on quickly and the UFC will market the fuck out of Conor.
It's plainly obvious at this point that they want Conor at the top. He is their biggest money maker, and Khabib has been hinting at retirement possibly as soon as 1 or 2 more fights.
So, what would you do? Set a precedent for other fights to run your business by pandering to the guy who has maybe a year or two left in him in Khabib, or back the guy who potentially has years left in him of big money fights and is by far the biggest draw the sport has ever seen in Conor?