Seriously, migration is as easy as pointing your MX records to them and running their import tool. It takes ten minutes (I thought switching off Gmail would take ages, but no, literal ten minutes).
I think the problem here is that this is a Gmail-specific, non-standard feature, meaning there is no specification that allows to implement this in a reliable, cross-client way.
However, Fastmail is working on something similar with JMAP.
I found this earlier HN comment to be really interesting: https://news.ycombinator.com/item?id=16372835
They are! [1]
"It really is coming in JMAP! Come along to IETF in London to have a play, or I'll be pushing out links to updated JMAP Proxy in the next couple of weeks and you can play with it there :)"
Once they've got JMAP nailed down, they'd just need to support Gmail's native API [2] for retrieving rich message data for their import process.