I don't think he would have necessarily been opposed to it, but it would have had to been the most perfect scenario to overcome the fact that Boston isn't the sexiest locale for lifestyle or his brand.
That scenario never came up because Boston has never been the best option for him when he was a free agent. The organization has also never been one to purposefully plan ahead for his availability because that often requires a lot of short-term pain in the hope that LeBron will pick your team. Unlike the Cavs, the Celtics also would not be as willing to give LeBron's crew so much access and influence within the organization. Imagine Rich Paul telling Ainge that they have to overpay this or that Klutch client.
Miami had the attractive location, the organizational pedigree, and most importantly, Wade already there. The 2nd Cavs go-round was logical because they had Irving already there, the draft rights to Wiggins, and he could rewrite the unhappy narrative from "The Decision." The last move to LA was logical because they had a lot of young assets and of course because it's Los Angeles where he has already planned his post-career life.
I'm sure there is also probably some residual animosity towards the franchise and fanbase for the adversarial relationship during the Pierce Garnett era. Those teams were LeBron's kryptonite for a few years and part of the reason he went to Miami was the goal of finally overcoming the Celts, which he accomplished. I'm not saying he hates Boston or the organization, but it probably didn't make sense for him to join the team he was trying to beat. I mean, the Celtics players really dogged him those few years he couldn't beat them.
Before the GSW rivalry, Boston was really always the big enemy for LeBron. Even after Miami, Boston helped ruin his last Cavs year by dealing for Kyrie. Two years later, LeBron and Klutch helped steer Davis to the Lakers (COLLUSION!) when everybody knew Boston was angling to deal for him. It's really kind of an untold part of LeBron's career narrative that he has constantly been competing with the Celtics both on the court and in orchestrating player movement.