What do you mean they cannot?
The UFC isn't some sports league. They can do whatever they want. If Khabib proves to be too much of a threat to their agenda, don't think for a second they wouldn't turn on him and strip him. They're not going to let some guy who only plans to stick around for a couple more fights set a precedent and decide how they run their organization.
They stripped GDR for not wanting to fight Cyborg because Cyborg had been done for PEDs and was coming off 2 wins at catchweight. Cyborg had no wins in the UFC at FW at all.
The UFC decide who the #1 contenders are, not us, not fight records, and not the media. If the UFC say Conor is now the #1 contender and next in line, then that's the way it is, regardless of how bullshit and unfair it is.