That is the thing: if we are ignoring durability, then Lebron is still the best player, but durability does matter, and after a long career of being crazy durable, LeBron's hasn't been quite as durable the last three years, so he gets dinged a little.
Consider this: the All-NBA team every year has 15 players (5 on the 1st, 2nd and 3rd teams).
Giannis, the obvious pick right now for best player, is one of only two players to be All-NBA 1st team both of the last two years (Luka the other). Durant wasn't on either team. So, Durant wasn't one of the 15 best players in the league either of the last two seasons, but we are supposed to buy him as the best player in the league now? Hell no.