The Sixers biggest problem is Embiid just can't stay healthy and when he leaves they just don't have anyone else that can effectively fill the #1 scoring role. Harris is best suited as a 2nd or 3rd option and Simmons should never be relied upon to be a scoring option at all. The Sixers need a real legit scoring option on their team outside of Embiid. If they could somehow upgrade Harris into a more consistent scorer, I think they'd probably win the title, but until they do that they are always going to struggle against the elite teams that can either cause Embiid trouble or that can really punish them when Embiid is on the bench.
As for Simmons, he is one of, if not the, best defenders in the sport. He is a superb passer and rebounder. He is a terrible scorer and shooter and always has been. The greatest disservice the Sixers did was in trying to make him a pure PG because doing that they needed him to score more and shoot more. Simmons is a PF and should always have been playing PF. They can run the offense through him, but they need a real PG on their team. They are doing a lot better this year by starting 2 actual guards in Curry and Green, which has helped, and we saw the result in the regular season. Ultimately I don't think the Curry and Green combo is good enough for the Sixers to win the title, and I expect them to lose to whomever wins the Brooklyn/Milwaukee series (I don't think the Hawks will really challenge them, but if Embiid's knee is an issue they could), but they are functioning better than they ever have with a more cohesive unit.
I actually think a Walker for Harris swap would be interesting for both teams. That would seemingly solve some of the issues that each team has. The Celtics need a guy capable of playing PF full time, but that wouldn't harm the way the offense functions. Harris is quick enough and big enough to play in the C's system. He is younger and healthier than Walker as well and he is clearly a similar level of scorer at this point so he could fit pretty seamlessly as the C's 3rd option (and occasional 2nd option). For the Sixers, Walker would take a lot more pressure off of both Embiid and Simmons than Harris does as he is a true scoring PG. I think he'd fit well there in that role.