I can see why people think the lad will go back and get into Chelsea's first team. I also think why some believe it would cost a huge amount to bring him back.
However when lads have been sent on loan 2 or 3 times and are above 22 years in age it generally doesn't tend to indicate much desire of the club in question to keep them. Chelsea are also not really sending players out on loan with an eye on letting them break into the team. It's not how they operate as a club, they don't really work to that sort of medium term planning with the manager. They get a new manager most seasons, who brings the players he wants to work in a short space of time and arranges them as appropriate. The funds are provided for this in then moving on decent prospects and younger players to help fund it.
There's almost a parallel operation going on there. The board operates transfers as a business, while the manager operates the transfers as a coach. It's clear to see who falls into which camp.
Historically they have allowed Salah, Lukaku, De Bruyne to all leave the club without much of an effort made to integrate them. While I think Zouma has the potential to be a fantastic defender, as an asset he will be worth a lot less to them than the other 3. If the price is right he will move on.
You also have to consider from Zouma's perspective, he is now back in the French squad, showing some of the best form of his career and looks to be moving to a club with more of a settled outlook with a manager who will give him time and patience to help him improve (completely the opposite of Chelsea). I cannot see him wanting to go back to Chelsea. He must look at many of the above players, and probably think, if he does 2-3 years with us he could well be set for a massive move to a top team on his terms. This is what Salah, Lukaku and De Bruyne did with Wolfsburg, Everton and Basel/Roma.
Given I can't see Chelsea massively wanting to play him, wanting to ideally sell him and the player wanting to come it looks obvious that a deal has a good probability of being done. I'd also say, it would probably be quite sensible of Everton to look to pay a reasonable fee for him, as opposed to hard balling it to the last minute. If we pressed enough I'm sure we could do another loan and possibly £15 million pounds as they won't want a sulking player, however I think settling towards £30 million may happen.
I suggest this, because Chelsea could become quite a fruitful partner for us to trade with. We tend to bring players of around 22-25 through very well, Brands seems to favour that range and Silva seems comfortable working with that grouping. Because Chelsea essentially stockpile young players, there will always be a conveyor belt of young players available that we should be in a very good position to utilise. There are already players like Loftus Cheek, Chalobolah, Mason Mount etc who are already piling up and could well jump at the chance of a move. If Everton establish themselves as a club who are good to work with it could work out well for us.
Either way, I'm very impressed with Zouma so far as I thought he would be. I anticipate him signing, like Lukaku did, not at a bargain price, but at a fair price and arguably more business of that nature being done.