If the batteries were user replaceable, it would have been a perfect story.
Sending the device back to the manufacturer (authorised service center) can be inconvenience(data formatting, TOTP apps etc.) at best to security breach(malware install, device imaging etc.) at worst.
Also, if indeed a user decides to replace the battery on their own, an iPhone is the least repairable phone out there and getting worse with every iteration.
Maybe we know a technician we trust, who can repair iPhone before our eyes, but alas independent repair shops doesn't get Apple love[1].
[1]https://www.macrumors.com/2020/02/06/apple-independent-repai...
Typing this on an iPhone SE
Wouldn't it be nice though if users could choose to patch security vulnerabilities without installing updates that deliberately slow down the phone?
Similarly, “updates that deliberately slow down the phone” sounds like a conspiracy theory. The closest we’ve come to that being real would have required a caveat “… when your battery has degraded to the point that the phone would otherwise crash”, which is an important distinction.
You must be just as friendly in person as online if you are one of the few people ever to have an Apple store employee not go the extra mile for you.