Removable batteries requires valuable space for the mechanism to hold the battery, transfer the power from the battery, eject the battery etc. It doesn't come for free.
At least based on what I've seen on camera mechanisms you're taking about 10-20% reduction in the battery cell.
Since the EU law is about making batteries "removable with normal tools" one could argue that encapsulating the battery like a camcorder battery with a plastic hull isn't needed.
A normal pouch with contacts will do.
Assuming "All day battery life" (24h) a 1% reduction in battery life is about equal to 15minutes but frankly I can't see the replaceable battery reduce the capacity at all if you are doing halfway good engineering - there is plenty of void space in an iPhone even now. By that I mean space over/under PCBs where the battery cannot go because it is still just a rectangular polygon even in say the iphone 15. I think it has 2 rectangular smaller cells with a casing around them to make an L shape.
So why doesn't apple forgo the "casing" and take that sweet 10 to 20% extra capacity? :)
The only point that you really list that is defensible is waterproofing. Lighter devices? Cheaper to manufacture? The idea that making a removable battery would make a phone noticeably heavier or impact the cost more than a cent or two is laughable.
Even with the waterproofing, I'm dumbfounded on how one hand people can laud Apple for their truly amazing engineering capabilities (I may question its utility but the Vision Pro is really an incredible piece of engineering), yet somehow a waterproof removable battery (which some other phones have) is beyond their grasp.