[mjl-]

> Any sane client will instead use UIDs for everything

Yes! Since last year there's the experimental UIDONLY extension that allows clients & servers to operate entirely without message sequence numbers. Saves quite a bit of accounting and possibly memory (for large mailboxes). It's a bit surprising the RFC so recent.

> [... thick vs thin clients ...]

For some programmatic email access, IMAP seems quite easy to use. Just write a programmer-readable line of text as command (eg fetch/search/store/etc) and parse the response lines (that's a bit trickier in many cases, probably needing a library, but you may be able to get away without it some of the time) and you're done.

About thick clients: Storage capacity is growing much harder than my email, also on mobile. I would be fine with thick clients that sync all messages. Allows for simpler protocols and simpler implementations. An email access protocol is then not much more than a two-way sync of email message files and associated "dynamic data" like flags/keywords.

But IMAP and JMAP are doing many things at once: synchronization for thick clients, and online access for thin clients. JMAP is specifically targeting the web (thin clients), so what was old is new again (although base IMAP isn't too helpful to (web-based) thin clients either).