We know that the modern /u:/ is not from that old /w/ because other words followed the same change - even words without /w/, like “moon”, “poop” (yup) or “boot”. In fact it’s how the digraph ⟨oo⟩ became associated with the sound.
The case of “hwo"→"who” is a bit more complicated. As you said the “wh” digraph used to be “hw”; the change happened in Early Modern times, and it was likely for readability - less sequential short strokes = easier to read. People around those times did other weird stuff like respelling “u” as “o”, as in the word “luue"→"loue” (modern “love”), for the same reason.
However, later on that /hw/ sequence of phonemes started merging into a single sound, [ʍ]: like [w] you round your lips to pronounce it, but like [h] you don’t vibrate your vocal folds. And if that [ʍ] happened before a rounded vowel - like [o:] or similar - it was reanalysed as a plain /h/. So for words like “who”, it’s like the “w” was dropped, just like in “two”, but in a really roundabout way.
And, before non-rounded vowels, that [ʍ] still survives in plenty dialects; for example, “when” as either [ʍɛn] or [wɛn]. This change is recent enough that you still have some speakers in NZ and USA who use [ʍ].
I guess my confusion comes from the fact I’m from a place that only within the last 100 years or so has all but lost the original “hw” in “correct” speech, except in “who”, and I was thinking that the ‘w’ had to have been preserved, especially if the ‘h’ was.
There’s also that the ‘w’ hasn’t vanished in “twenty” (or “twain”, etc.).
We know that the modern /u:/ is not from that old /w/ because other words followed the same change - even words without /w/, like “moon”, “poop” (yup) or “boot”. In fact it’s how the digraph ⟨oo⟩ became associated with the sound.
The case of “hwo"→"who” is a bit more complicated. As you said the “wh” digraph used to be “hw”; the change happened in Early Modern times, and it was likely for readability - less sequential short strokes = easier to read. People around those times did other weird stuff like respelling “u” as “o”, as in the word “luue"→"loue” (modern “love”), for the same reason.
However, later on that /hw/ sequence of phonemes started merging into a single sound, [ʍ]: like [w] you round your lips to pronounce it, but like [h] you don’t vibrate your vocal folds. And if that [ʍ] happened before a rounded vowel - like [o:] or similar - it was reanalysed as a plain /h/. So for words like “who”, it’s like the “w” was dropped, just like in “two”, but in a really roundabout way.
And, before non-rounded vowels, that [ʍ] still survives in plenty dialects; for example, “when” as either [ʍɛn] or [wɛn]. This change is recent enough that you still have some speakers in NZ and USA who use [ʍ].
I guess my confusion comes from the fact I’m from a place that only within the last 100 years or so has all but lost the original “hw” in “correct” speech, except in “who”, and I was thinking that the ‘w’ had to have been preserved, especially if the ‘h’ was.
There’s also that the ‘w’ hasn’t vanished in “twenty” (or “twain”, etc.).