I guess everyone is replaceable in the end. That said, if the two best WW in the world are replaced with lower ranked guys they already beat, You can't exactly argue that your organization is the best in the world. Something the UFC boasts about all the time. Dana is always bragging about being number 1 and having the best guys. If he were to just replace em, then all that credibility gets flushed down the toilet. It also would not help attracting new guys to the sport if they keep treating their champs like chumps. If they are just replaced then the whole organization loses credibility. It is bad enough the Bellator org is currently holding a tournament where the winner gets 1 millions dollars and has some pretty good guys. So its a tough pickle the UFC is in. Gotta pay up and bite the bullet or eat crow and be known as the second best org in the world.
If they lose Usman and Colby to Bellator, How does that look to a kid who dreams about being the best WW fighter in the world? He looks at UFC's roster and pay and compares it to Bellator, then quickly comes to the conclusion that to be the best in the world, He must go to Bellator and beat their champion. Sure business is business but bad decisions have long term social and economical effects. It is the same with other sports like Basketball, Hockey and football, When a organization loses its stars because they refuse to pay em. They fall from grace and get bumped outta contention.