Ben is legit elite at 2 aspects of basketball. Defense, and playmaking. Not a single one of Siakam's skills even come close to either of Ben in these 2 things.
Yes, Ben cant/wont shoot and it's a challenge, but first, tons of players his age could not shoot and then did. All he needs to do to become a top 10 NBA player is develop a decent enough shot where teams must consider it a possibility.
Lastly, he's not a PG, and was always a poor fit with Embiid, here, if you keep Kyle by going over the tax, Ben becomes our playmaker from the PF spot and has more shooters ( OG, FVV, Kyle) around him than he ever did in Philly. We also potentially become elite defensively.