Perhaps higher than we all think. Recent history and common sense would dictate 95% plus confidence in Joel leaving. And he very well might. As Jay Bilas was talking during the OU game about his opinion that Joel should stay another year (or two I think he actually said) I sarcastically laughed and said "yeah, right." But something he said after that caught my attention. I believe he said that Joel has been gathering the opinion of professional big men who he respects and they've been suggesting too that he stay a little longer. Bilas' take is that Embiid is "the real deal." He doesn't have to strike while the iron is hot; or fool anybody with a stat-inflated flash-in-the-pan year. He will get the benefit of the doubt from GM's even next year if he stalls out and is not showing progress because he has all the physical tools he needs to succeed.
Other factors include his culture as well as his personal/family situation. He is not in a position where his family is in financial dire straits and need him to be the bread winner. In fact his father, The Colonel, whom we can infer values structure and discipline, may very well approach his development looking at the big picture and realize that a year of getting bigger and stronger won't necessarily improve his draft stock, but will better prepare him for the NBA.
Look. I'm under no illusion that he's staying. I don't think it's probable. I don't think it's likely. Chad Ford reported last night that in something like 13 out of 14 scenarios Embiid would be the first pick overall. With injuries to consider, I would not blame him one bit for going. Shoot, I'd go. I guess the point of this post is to suggest its not a done deal and he may actually be considering staying more than I thought he would. What are the odds of Embiid leaving? After consideration of these points, my heart says 50%. My head says 94%.