At five years you're probably going to be disappointed with the battery performance, but how many people are continuing to use a 5-year-old phone?

My brother has a 6+ years old Iphone, my parents both have a 5 years old Samsung Galaxy Phone (S21 and A51). None of them complains.

On top of that, giving the user access to the battery means the phone body can't be fully sealed against moisture and dust

Do you think computer waterblocks are sealed using glue? They aren't. Screws and a good old o-ring are all you need to make a repairable AND waterproof phone. But they don't want a repairable phone, they want you to buy a new one whenever possible.

plus the access panel is a big mechanical weakpoint which means the body will be less rigid than a fully enclosed device and thus more prone to breaking when dropped or sat on.

I dropped my FP5 multiple time. He never broke. My brother IPhone got a shatered back, and he had to replace the screen once for falling from a distance the FP5 just shrug off.

Plus, the addional space required to fit in the replaceable battery casing, the removable access panel and the contact points for the battery means either the whole device will have to be bulkier or the battery will have to be smaller (than it would otherwise be with a permanent internal battery).

True, but ot also don't have to be the old pogo pin way. Any currently available battery is a removable battery given it is user accessible and isn't glued to the board.

Today it's weight and bulk, plus fragility that will probably lead to equivalent or increased e-waste.

I wonder... What would be the biggest e-waste? A dead battery or a dead battery with a whole perfectly functional phone attached to it?