Oh, I think Embiiid holds most of the cards most of the cards, though at the same I also don't think there's any scenario in which he turns down the extension, because if he gets hurt next year and doesn't make all-NBA he's leaving money on the table, and only the Sixers can pay him the most money ever anyway.
He will be pushing the Sixers to do it no matter what, and if the Sixers don't do it, and also don't fix things this season, he'll be pushing for a trade. The question is is that really a bad thing for the Sixers compared to committing even more money to him as he ages.
But such a scenario is also reason to still hold on to Ben. At 24 years old, he may not "fully develop" until he's 27 or 28, just like Lebron and Magic. He's just got a much lower current ceiling, and the Sixers may have a much shorter championship window with Joel