I'm not sure if that's the chicken or the egg, these protocols (RDP, ICA) have been deployed for more than 20 years and I'm not sure that the standards you references aren't just ex post facto documentation of existing practice.
> the usual set of rendering operations, font and image caching, and so on
One more thing to point out is that all the protocols I listed are add-ons to an existing system (mostly MS Windows) that isn't designed around remoting applications and so has to integrate with an infrastructure designed around local display, the same kind of problems that would need to be solved to remote native Wayland applications.
The methods for local display and remote display have little relation to one another and I think experience has shown that this is a good arrangement, trying to do both with the same method leads to a design that is poor for both cases.
Does any of that sound similar to your understanding of the situation? 8-)