Are you.... sure about this? Like how sure are you? What do you think you know?
The human body changes surprisingly fast. Astronauts that stay in outer space for too long can't even walk when they return to earth. That "too long" is like... a year basically.
A "lifetime" of muscle mass will last for how long once someone starts taking androgen blockers then estrogen?
The reality is while we think of 'sex' as being some kind of chromosomal thing, that isn't what really separates the 'genders'. After all, apart from genitals, there is little external difference in babies. So it isn't exactly the XY/XX... of course one might say "but that happens during puberty", which is what exactly? The significant rise of sex-hormones. As the saying goes "if you can't make your own, store bought is fine" (regarding neuro-transmitters, sex hormones, etc).
So, are you SURE that an individual that "lived in the prime of their life as a man" is just as strong as they once were once they have spent years on hormone therapy? If you're sure, why are you sure? "I don't think it's fair" ... but why? Is is fair to trans athletes?