If you're encoding data for a domain guaranteed to contain only US English words and US English symbols, and you're OK without the occasional non-ASCII bits in

If you're encoding data for a domain guaranteed to contain only US English words and US English symbols, and you're OK without the occasional non-ASCII bits in US English like diacritics in loan words, then sure. But such domains are vanishingly rare. As for your examples, "most of the internet" might still contain non-ASCII, I think, but even a little bit of the internet is still a gigantic and highly active part that's inseparable from the larger domain of "the internet". And, while the legal symbols in the syntax of a programming language might only be ASCII, programming languages must process arbitrary strings, which may be user-facing, or be in some other domain where ASCII is inadequate.