When you start the fight out-boxing someone, then begin walking your opponent down it mentally dismantles the fighter, breaking his will. A man's defense and speed culminates into a level of elusiveness you have never witnessed. You keep getting countered and then by the 5th round, he is so confident he turns the tables on you and starts walking you down. Were the fighters of previous eras mentally tougher?
also, atlas argued to hopkins that fighters from previous eras wouldnt be shocked that an older man is as good as he is.
Two main factors; Mayweather being an exceptional fighter, and there being a lack of comparable talent from 140-154. 15-20 years ago, you had the likes of Trinidad, Quartey, Whitaker, De La Hoya and Mosley, to name a few, floating around that weight range. All very good fighters, all comparable to one another in overall ability. Outside of Pacquiao 3-4 years ago, Mayweather hasn't had that, and what we're seeing now (along with some pretty ordinary matchmaking) is the result.
Pretty much this. Remember an old De La Hoya was very competitive against a prime Mayweather and a completely shot Mosley had him very hurt in round 2. Mayweather is partially the product of coming along at the right time like Marciano only Marciano fought everyone while Mayweather has some big names missing from his resume. An exceptional fighter no doubt but also the beneficiary of circumstance. As for your point on defense sapping another man's will to fight it can happen Willie Pep maybe the best ever at it won many a fight that way.