One thing to the Mocking framework: https://mswjs.io
it looks indeed very interesting.
In GWT I used to implement the mock implementation in "development time" and remove it in "production time" by using GWT options for different "sourcepath" and "entry point" with help of DI. It works very well. So I never have to run the "server / REST APIs" part to be able to develop the whole UI. Very productive.
.... and to try to answer your question: "what are the obstacles for you to use JS or TS (possibly mixed with Java through J2Cl) for frontend development?" here you are: 😉
(1) JS sofar is difficult to use for a bit complex UI.
Quote from Anders Hejlsberg:
The reason: "Theia is a framework to build web IDEs. It is built in TypeScript and will give contributors a more enjoyable experience with a programming model that is more flexible and easier to use, and makes it faster to deliver their new plugins." But I'm not sure why they could not build plugins based on GWT which is flexible and easier to use. As we know we all like to begin from scratch and don't like the idea of maintenance 😀😅
Introducing TypeScript to a Java team is a different story. The language is the smallest problem but the ecosystem, library, build, CI/CD pipeline, dependencies management, artifacts scanner, etc.(see the presentation of Netflix above). For many companies you mostly have some intelligent forms and for this purpose I don't see that TypeScript has advantages in comparison with Java / GWT / J2CL (of course we need to address the UI frameworks based on TS or GWT / J2CL as well). One thing you should not forget, on-boarding process of new developers and the ability to be a full-stack team. A team which all the developers can do everything, from frontend to server-side implementation (not to have separation between frontend and backend developers).
Just my 2 ct.