Earlier quoted context omitted.
Yeah, it's kind of funny when you look at what GWT had vs what modern JS development looks like. Multiple browser targets, using the same language on both client and server, static types, RPC declarations, a dev mode server, etc. That said, GWT-RPC also went from being nice to a pain. The serialization formats were "proprietary", in the sense that they were GWT-specific and couldn't be reused with any other platform.…
I agree the implementation of GWT-RPC could have been better. It was also pretty slow if the payload was a lot of tiny objects. But I'm less enthused with promises than you are; I like GWT-RPC's programming model. I'm surprised some sort of IDL-based client & server generator for REST services hasn't taken off.
The big problem with an IDL that people want to avoid is tight coupling, IOW, all systems have to be upgraded at once to use a change in API.