I think we need to take a step back and revisit what Sandcastle is all about (and what it isn't).
Sandcastle is first and foremost a tool for interactively exploring Cesium examples, allowing people to tweak the code or otherwise learn the API. Secondly, it's used to showcase Cesium capabilities in some more complicated examples not strictly designed for learning.
Sandcastle was never meant to be a full IDE or tool for developing production Cesium applications.
As it stands today, Sandcastle accomplishes its goals pretty well, but of course there's plenty of room for improvement. We identified 3 key issues for the Google Summer of Code project that we wanted to fix:
1. It's fairly useless on small screens and should be refactored to be responsive.
2. The overall look and feel is dated and it would be great if it it had a more modern looking UI.
3. It depends on the Dojo toolkit, which is a bit heavy and not used elsewhere in Cesium. We wanted to remove this dependency and replace it with libraries already used elsewhere in the code, like Knockout and AMD.
Adding additional third party libraries like React, JSX, and pubsub seem to be in opposition to number 3 above, so there would need to be really good reasons to include them, especially if other libraries in the Cesium repository can already accomplish the same thing. If such reasons do exist, I would not be against adding the dependencies to Sandcastle. However, I don't see anything in that currently warrants it. In response to your specific bullet points:
1. Sandcastle is actually a fairly simple application. I don't understand how composability or modularity help it. What specific problems are you trying to solve that these features would help with?
2. Sandcastle is currently designed to work completely client side and even if we had a server-side component, we have no plans to support online collaborative editing. Even if this were to change, there are numerous ways to implement such a feature and choosing React simply because there might be a solution that we can use for it doesn't make much sense to me.
3. An isomorphic framework would provide no benefit to Sandcastle. As I already stated, there is no server-side component. Furthermore, the client side rendering time in Sandcastle is the Cesium code itself (which has to be done on the client due to the nature of WebGL) and not the Sandcastle user interface.
I'll also add that the choice of libraries is not just a purely technical one. Sandcastle needs to be maintained and supported along with the rest of Cesium and each additional library or feature burdens the developers with the need to understand and support them. Keeping Sandcastle simple and similar to the rest of Cesium makes it easier to maintain.
Hopefully some of the other committees can respond with their own thoughts on the subject.