Not even just non English speakers, we in en-gb land had enough trouble getting £ to work 100% of the time, which was kind of a basic requirement for business use.

It's not a basic requirment, just convenient. In the absence of £, GBP works nearly as well.

Didn’t extended ASCII cover that and make text terminals usable with non-English character sets? Genuinely curious.

Extended ASCII was actually several different and incompatible encoding, "codepages". It was kind of a solution, but there were major issues that made it much worse than Unicode/utf8:

- Asian languages needed special encodings, 256 characters weren't sufficient.

- It was impossible to work with different languages in the same document, each codepage only covered some character/alphabet.

- There was no 'safe subset': Unicode codepoints 0-127 are exactly ASCII, which means that ASCII is compliant utf8. The same wasn't true at the time.

How would you use extended-ascii to write Arabic?

The following was actually used:

https://www.charset.org/charsets/iso-8859-6

There’s no single byte encoding for Chinese, Japanese, or Korean though.

In practice, young folks write arabic with roman script when they chat. (with numbers to supplement the missing letters)