What are they looking for (in your experience)?
In my experience, most people use an app (website) to solve some problem (buy something, pay taxes, whatever). They care more about functionality than how smooth the loading animation and transition was. Progressive enhancement seems like a very good way to build something people actually use (and rely on).
There's nothing wrong with using JavaScript. There's nothing wrong with depending on JavaScript for specific functionality. However, I don't think it's acceptable to completely break down if that JavaScript fails to load.
With SPAs and "modern" web development practices, it's all or nothing. Either everything works, or nothing works.
Not with things I build.
I think you may be overthinking the appearance aspect of a failure state. The application doesn't need to look the same when CSS fails to load. However, the application should be functional.
This thinking carries over to the backend as well. My application server doesn't require all services to be up and running. Instead, it's able to query and tell what is working and what isn't. That information is then bubbled down to the UI in one way or another. That may mean certain functionality is unavailable (ex, if solr isn't reachable then search is disabled).
I've never liked where those code bases ultimately end up, which seems to be just a twisted maze of hacky solutions to make a bunch of poorly aligned code to work together.