Simmons with Saric and a center actually has a chance to work for a few minutes per game. Simmons should be able to guard opposing SFs a bit and Saric should be a pretty decent shooter from 3, which would space the floor.
When they're not out running in transition, the Sixers bread and butter will be the Simmons pick and roll with Embiid or Noel with three shooters around them. This is really intriguing because opposing bigs aren't used to being in that situation at the same time.
These are some incredibly important distinctions that tend to get glossed over in a general what position can he defend topic.
There's a world of difference between getting 10 and 30 minutes a night there. The former makes a ton of sense. The lack of spacing, especially after Hornsby went down, makes his ability to run the pick and roll as the primary ball handler tough to identify with the tape we have, but he was not as proficient this year as you would expect from someone with his skillset. If you want Simmons, you have to blame the spacing in no small part for it. The trick then becomes making sure you have a 4 who can shoot and a true rim protector at the 5 because a Simmons/Saric 3/4 stack is not exactly quick-footed.
I just don't understand why some of the same people who were incensed that Noel was being played out of position now want to put Simmons in a situation where you're clearly minimizing his current strengths and magnifying his current weaknesses. Put him in the mid post where he's comfortable and let him develop. Maybe one day down the line he has the skills to defend the 3 for 30 minutes a night, but he's not there yet.