The problem with the Celtics trading for LeBron is that to match salaries you have to deal a lot of that depth, or you have to deal Horford.
LeBron is on tap to make 33,285,709. Even if you clear his salary, the Cavs can only take on $5 million without going back over the cap. So even if the Celtics can take on LeBron without being over the cap (also difficult), you still have to match salaries. As well, what's the point for Gilbert, other than being insane, to pay luxury tax without LeBron? (Yes, I understand the "get something for him" aspect, and yes, sure, you might also have a fire sale and clear Love and Thompson for a reset)