From what I’ve seen so far, there is some line that balances between too much concatenation and too little. Unfortunately I think that line is going to vary from site to site. If we are lucky, we might get some good guidelines ( always concat if loading fewer than X bytes ) that can be applied by tools and services automatically.
Having hard rules that “always make things better 100% of the time” are becoming less common in the world of HTTP/2, modern browsers, and mobile clients. There is no substitute for making measurements and comparing the results.