Why are GOOSE and TUNE analysed as having the same vowel?
So goose and tune both have [uː ~ uw], but tune, as well as many (most? all?) other long-u words, originate from Norman French through Middle English /yː/ which evolved into [juː]. They merged into words that used to have [ɪw] and [ew] like new and threw. Later the [juː] was affected by yod-dropping and yod-coalescence in different dialects, so you get [nuw ~ njuː] and [tuːn ~ tjuːn ~ t͡ʃuːn] which only merges with goose/toon words under yod-dropping, since toon is also [tuːn].
My question is why is the [j] in the TUNE set (and I guess in CURE as well if you split it from a hypothetical TOUR set) is considered part of the onset consonant cluster and not a rising diphthong in the nucleus. Is the reason etymological, phonological, something else? It's odd to me that long U represents a two-phoneme sequence when other diphthongs like /o͡ʊ/ and /a͡ɪ/ are treated as a single vowel phoneme?
E: just realised I posted this to the humour sub, meh. Hopefully you guys can still help but feel free to jerk instead