For me it's down to his fighting style. I remember when he was a young guy, full of enthusiasm for striking, running out there without fear and just standing and banging with guys with almost limitless energy. Then he got old, he was still striking but was losing, he was still standing and banging but was running out of energy quickly. Now he's even older, but he's back to fighting how he was as a young guy and the only possible reasons for that it's because of the TRT or that he has access to a fully working time machine. I also don't like how the UFC up until this point have deliberately kept him out of the USA to avoid having any complications.
My other issue, and it's the same issue I have with anyone who uses TRT, is that there's a reason he's got low testosterone levels, either because they're naturally producing less testosterone because they're older, or because of using added testosterone or other steroids in the past. People who have bad vision aren't allowed to wear glasses when they fight. People who have a lost a leg can't go in there with a cybernetic leg. People who are short of breath aren't allowed an oxygen tank in there. Admittedly those are extreme examples, but why should someone with a naturally weaker system be allowed to top themselves up to the level they had when they were younger?