#4. Smaller payday
Fighting Conor McGregor in any capacity is a good way of ensuring you receive the biggest payday of your career. However, Charles Oliveira would likely not earn nearly as much if he were to fight McGregor at 170 lbs next, rather than fighting him for the title at 155 lbs.
Champions in the UFC receive a share of the PPV sales from the events they compete in. Oliveira had been enjoying this benefit but was stripped of the belt at UFC 274. Dana White then stated that he wasn't sure if 'Du Bronx' would continue to receive PPV money moving forward as he was no longer the champion.
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Fighting Conor McGregor would ensure a big payday and huge sales regardless of the weight class. However, Oliveira would make far more money from this bout if he were still receiving PPV money too. He may be better served reclaiming the 155lbs belt and then taking on McGregor for the title at that weight later on.
#3. It doesn't make sense rankings wise
The UFC rankings system is often mocked as it does tend to be ignored when there is the opportunity to put a big fight together or push a star. Just recently, we saw an unranked Alex Pereira take on No. 4 Sean Strickland. But pitting the No. 1 lightweight against the No. 12 lightweight in a fight at welterweight is a new level.
If we put the lack of sense from that perspective to one side, it could have serious implications for the career of Charles Oliveira. While Conor McGregor is a big enough star that the rankings don't seem to apply to him, Charles Oliveira is not in the same position.
Much of his appeal comes from him being the top lightweight in the sport and even if he were to lose at welterweight, he would likely lose that status. McGregor would likely take his spot in the rankings with a win. Dropping down the lightweight rankings for a fight in a different division would be terrible for Oliveira's career.