I have no idea how to fix parity in the NBA. But the NBA’s main tool has failed.
There is no fix, unless you make it so that the best of the best players aren't allowed to stay with a team for very long.
Guys who are in the conversation for top 10-15 of all time (especially those in the GOAT convo) will at some point end up in a situation where they have enough talent around them to start racking up championships, and most times if you're good enough one year you're good enough for a couple more. Pretty much the only obstacle for guys like that is managerial incompetence, injuries, or entering the league at the same time as another great player with a better team around them.
I mean, how would you have kept MJ's Bulls from dominating the 90's? Or Magic and Bird from owning the 80s? Maybe you could have artificially dismantled those teams, but no matter where they went those were still championship centerpiece type players that would quickly be built around. You build around guys like Shaq, Kobe, Duncan, Lebron, etc., sooner or later you're going to have a championship caliber team. Championships follow those guys around for a reason.
Besides, parity is overrated. The 80s were when the NBA exploded in popularity, when there was no parity at all. Whereas the 70s which had 10 different champs in 10 years (can't ask for much more parity than that) pretty much was the NBA at its crappiest. If you'd shuffled Bird and Magic off to other teams in the 80s, all that would have probably happened is those guys in different uniforms still fighting for championships.