I actually prefer no syntax highlighting, and I certainly wouldn't make any claims about it being useful. But something being "useful" is often personal - I find IDEs useful, others find Vim useful, maybe one is better or worse than the other or maybe we're all different and our brains function in different ways and that explains the difference.

With assembly versus say, Go for writing a web server? That's trivially observable, good luck arguing against that one.

That's the whole point. The sky is blue is trivially observable. Any claim that someone has disproven something that is trivially observable should be met with skepticism.

If you have something that needs to be done, and an agent goes and does the whole thing for you without mistakes, it is trivially observable that that is useful. That is the definition of usefulness.