His test will come when he becomes a free agent. This is where LeBron failed miserably. The Decision was maybe the worst decision one could make because even after all of LeBron's accomplishments, that infamous decision will forever loom over his legacy. Not the fact he left for Miami, but how he did it, how he announced it.
Not really. Most people have moved on from The Decision. Even those who truly don't like him, rarely mention it anymore. Lebron has even admitted that he should have done it differently.
He caught a lot of flak for several years (and deservedly so) for how he handled that free agency.
As much of a bad look as "The Decision" was, it was him using his power as a top level player to team up with two other top level players that most people had an issue with. That's the thing that some folks still cling to when criticizing him.
Since then however, we've seen more players handle their free agency in a similar fashion. It's to the point now where we expect it.
I'm not saying that it's not part of his legacy. It is and always will be. But I doubt it will do near the damage to it that you suggest.
In fact, at least from a "pro-player" perspective, what he did was actually a good thing because he showed the players that it was possible to take some control over where they spend their careers.