It's certainly surprising that Simmons hasn't improved, and has actually regressed, particularly as a shooter. That wasn't particularly foreseeable. I do think that the possibility of him never really being a good fit alongside Joel Embiid was foreseeable though, and Philly should be called out for never, to this point, maximizing the roster around Embiid. I think that choice was less about optimism that Simmons would eventually be a good shooter, and more about trying to hedge against an Embiid injury. Having Simmons around enabled Philly to have a backup superstar if Embiid went down. They could theoretically recalibrate the team, surround Simmons with shooters, and carry on. Trading Simmons, even at peak value, would probably not have returned someone with the same upside, even if they would have fit better with Embiid, so Philly stuck with Simmons.
Point being: Philly should have recognized that Joel Embiid was their meal ticket to a championship, their once-in-a-generation foundational superstar, and oriented the entire roster around him years ago. But they didn't, and now it's too late.