I'm not saying your performance don't drop but thays nature and even tho your physical ability lessens you have a lifetime of experience
There's a point reached where the physical decline is so profound it outweighs the reservoir of experience built up by a fighter. People talk about fighters 'ageing overnight' for a reason - when watching a boxer who, because of age and subsequent physical decline, can't athletically deliver what that experience informs him he should do. The biggest clue is when they can't let their hands go any more due to that split second of delay between wanting to deliver a punch and being able to physically respond to that subconscious instruction. It's called muscle memory and yes, that is the result of years of training. But if the body can't respond as required, that training is rendered relatively meaningless. These are elite athletes, executing a ridiculously difficult technical skill. We're talking fine margins and the blurring of those margins can often be down to getting old. It's a thing...
Sometimes fighters only find out their body can't deliver during a fight and not before, so it can't be 'on them' until their decline is confirmed in the heat of battle. If they carry on after such a performance then I agree, it's on them. But most fighters don't make a fortune from boxing and it's all they know.
I think it's a legit excuse if uttered before the fight, but not after it. Lots of people thought Hopkins would spank Calzaghe. After Calzaghe beat him Hopkins suddenly became old. He went on to compete successfully as an increasingly older man for a good number of years, gaining a ton of credit for doing so. Yet you'll still hear people say Calzaghe beat an 'old man'. Can't have it both ways. On the other hand, taking on a fighter who's clearly way past it and beating them as expected is no feat, and shouldn't be feted as such. It's all about the prior expectations IMO.