Well, how do you establish what an individual's original baseline (before the age-related "drop") was? What's to say that replacement doesn't enhance them beyond their natural levels.
IMO, depleted testosterone levels are the same as anything else "lost" due to age (speed, quickness, etc.). I'm OK with people using the stuff, as long as they're not fighting professionally.
Perhaps these guys just need to admit they're getting old, have lost what they once had, and retire, just like most athletes do in their mid-late 30s.
That brings up another question though - Why do combat sports athletes seem to try to "hang on" longer than other athletes? Boxing and MMA would have to have a higher percentage of people still participating at 35+ as any professional sport, theoretically past their physical prime.