IMO, it's not a problem if your goal is to have fun, meaning what's dangerous is if your goal in gambling is to win by choosing retired athletes. After all, usually when senior athletes return to fight, it's usually just for fun, to remember their past, like a friendly match, and if we watch and bet on it, our intention should also be just to have fun, to enliven the event. That also means if you don't want to lose money, it's better not to bet, or switch your bets to other matches where the athletes are still actively competing, unless you don't mind losing money, that's all.