Right, so they could get everything those two players provide in one and then have another player added to the guys on the floor who can do different things. That would make CJ valuable to them. Now Maxey might end up being able to do what CJ does but not for a little while, so while they're waiting to see how that pans out CJ can be a big deal for them. They don't have anyone else that does what CJ does, as you just pointed out they're starting two guys that when combined do what he does right now. It's not a hard concept. If you think that Maxey or Seth are as good as or better than CJ at what CJ does then you're right, the Sixers have no use for CJ but if you are evaluating their abilities correctly you know that's not the case.
The Sixers would be so much better if they were starting CJ and a defensive specialist in the back court, while bringing Seth and Maxey off the bench... so much better. Does it make them better enough to equal the value that Ben Simmons would bring to us? No that's not really close and is why 2 or even 3 first rounders or other players would probably be necessary to make that trade work but don't get it twisted CJ would make them a helluva lot better than they are as currently constructed.