> but I also have to believe the designers at Apple are trying to make things better
Except they're not trying to make things better. They're just changing things - often ignoring their own UI guidelines to do so - and introducing bugs that cause data loss at the same time.
Updates shouldn't cause data loss - especially for something as critical as email.
You are correct, updating the UI and the underlying storage mechanism are almost certainly separate things falling under a broader umbrella of "lets change up Mail instead of fixing long-standing bugs".
I would really love to see every other major release of macOS and iOS to be a bug-fix release rather than a "lets change all the things" release.
Except they're not trying to make things better. They're just changing things - often ignoring their own UI guidelines to do so - and introducing bugs that cause data loss at the same time.
Updates shouldn't cause data loss - especially for something as critical as email.