I think AD simply does not want to wait any longer to be a playoff contender. Zion (like most star rookies) is still likely 2-3 years away from being a great player in the NBA.
While people are currently enjoying dancing on LeBron's corpse, he still averaged 27 pts, 8 rebs, and 8 assists last year. Additionally, one of the effects of the Lakers surrounding LeBron with bad players last season is that they would still have a max slot even if they traded for AD now. If they make the trade, that would at least be enough to attract a player like Kemba Walker or Jimmy Butler.
So the attraction from a basketball standpoint would be LeBron + AD + another all star.