Why not simply make the battery replacable?
Solid construction and mechanical fasteners are a feature. Servicing any part of an iPhone is not that hard; lack of serviceability is a myth.
Additionally, batteries are not the most commonly replaced part. So far my SE has had four screens and a lightning connector. I'll do the battery myself when I notice performance degrade, too.
have non-replaceable ram and cpu is one thing, but a part that is known to deteriorate with age and usage should be user replaceable.