Well, that's the question isn't it? Seems to me it might have been more like a Jimmy/Ben issue than a Jimmy/Joel issue. That would be the juiciest possible scenario - Simmons did not want to take a backseat to Butler, Simmons is the Sixers' future, and they stuck with him and pivoted to Horford (Harris was probably coming back regardless though there was more pressure to do so once they lost Butler).
Because if it was just Butler being selfish (or even critical) I think the Sixers would have let that leak out. If Brown didn't like Butler in the room badly enough to let him go they would have let that leak out (the other stuff did).
But what I just posted could be complete nonsense, and so, IMO, is most of the other speculation about Simmons (i.e. that he doesn't work hard enough or spends too much time having fun in Hollywood). I think he does work hard, and has also mostly done anything the coach has asked of him (it was only this year that Brown finally started talking about wanting him to shoot). But he and Brown might simply have too friendly a relationship where a stick might work better.
And I certainly don't think you can accuse Ben of disappearing. He stepped aside to let Butler be the primary ballhandler last year, and they don't even get to Game 7 against Toronto without his defense. And now he's stepped aside again because Horford didn't work out.
Was already writing this before your other post: 2018 I don't even think about, he was a rookie. It might take 4-5 years for Ben to become a dominant player. It took Magic and LeBron that long. But by abandoning the process and blowing the Fultz pick (and then every move since) neither the front office nor the fan base can wait that long.