JMAP sounds like a crutch for mobile devices that can't keep a TCP connection open. The internet is slowly being gimped to support mobile devices.
JSON protocols have completely taken over the market because they are massively easier to deal with and trivially parse in to structs which don't require a 200 page document to detail the protocol.
You can write a JSON API that's equally bad, doesn't support paging, doesn't let you select just the fields you need, is missing key information in the extracted fields forcing you to parse the entire RFC822 text as a fallback, etc. Dealing with the parser is just one part of the battle.
Complexity has exploded as of late. It used to be a joke that programs grew until they became able to read mail. These days programs grow until they contain a web engine. It's not enough anymore to just have a chat window, the chat window has to generate thumbnails of webpages for you.
We would be better off having a smaller selection of better software over 100 different email server implementations which can't agree on a basic set of features gmail had a decade ago.
You can't "give some grace period of compatibility" in an open / distributed ecosystem, every version of the protocol lives forever.
YAML's dumb fuzzy booleans still cause issues all the time even though Yaml 1.2 was released 13 years ago.
Nevertheless, a valid current-day example for non-mobile devices requiring a tolerance to dropped connections: I regularly dock and undock my laptop when heading to meeting rooms, and simply switching from docked Ethernet to WiFi causes a network switch that breaks a lot of apps. Engineering tolerance for this kind of thing is only good.
Reality is people use multiple different and mobile devices, roam across connections, and intermittently go offline. Building software and protocols that are designed around this is beneficial to every use case - even those using whatever you consider a non-"terrible computer" to be.
When I worked in an office, I had the same issue as the OP - docking/undocking my laptop resulted in switching from wired to wireless, and connections (eg: SSH, RDP) would often drop. I ended up unplugging the ethernet at my desk and using wireless most of the time because it was just easier.
What would help is protocols actually support the concept of a "reconnection" -- ie: a TCP session that has the same identifier or token as a previous one that lets it it silently resume instead of starting fresh with new authentication. This isn't even that hard to implement if designed in from the start.