Why I'm not so sure LeBron's self-management is that incredible

First of all, I do think LeBron's ability to manage himself is legendary.
But if you watch videos of LeBron from his Miami days 15 years ago,
he had a larger physique than now, unbelievable speed, and used his touch to tear through the rim..
During his second stint in Cleveland, his athleticism declined slightly, but he reigned as a top-level player by covering it with his BQ.
In his 22-23 seasons with the Lakers, he performed as a #1 option, averaging 30 points, but since then, his performance has definitely declined significantly. Though, did he prove he's still top-level during last year's playoffs?
Honestly, I had somewhat accepted the idea of evaluating LeBron's longevity based on his legendary self-management..
But after watching the Miami era, it seems like his overall level has just dropped significantly.. Since he was such an overwhelmingly physical player to begin with, perhaps he's only competing at a top level at forty because of that..
To be honest, while LeBron's self-management is excellent, I don't think it played such a massive role.
As a side note, I'm curious what LeBron from that era would be like if he were placed in the 2020s, where fast breaks are rampant. Even last season at age 41, he was dominant in transition situations, so I wonder how he would have been in his 20s.