Nobody is arguing that you should keep a phone longer "just" because the battery can be replaced. You should keep a phone for longer because you want to keep the phone for longer. With a replaceable battery, you have that option.

What would convince you to keep a phone significantly longer?

Better phones stop being made.

Which new recent features are so important for you?

No dynamic island, periscope telephoto lens, USB-C. We are far from peak phone.

That happened a good few years ago really.

For a while, phones were gaining 50-100% performance per year and noticably better screens+cameras. Now it's down to a few percent and minor camera updates are the main selling point.

Yeah, as I mentioned in the edit, we should also have a deposit for the replaceable battery, but at least it could be smaller, since it is just for the battery.

Or you give the phone to someone that wants or needs the phone.

Reusability is about not being selfish.