Hangul's just as flexible as any other script, and would work - or at least could work - as well as Latin.
I disagree. How could one use Hangul to write sounds which don't occur in Korean, such as radical consonants? What about vowels other than those of Korean like [ʉ] or [ɒ]? With Latin there's already a myriad of letters & diacritics encoded in Unicode to write any sounds of any language.
The same way Latin does, through modification, diacritics, and digraphs. After all, Cia-Cia has a /v/, which is not found in Korean. Remember, Latin originally had only 20 letters. It was expanded over time--G is just C with a cross-bar diacritic. Korean originally had 48 letters (many that were used only for Chinese are now obsolete, like the one resurrected for Cia-Cia /v/), plus at least 55 vowel digraphs and trigraphs like modern ae
. You could do a lot with a system like that. And that's not even counting consonant digraphs - I count 78 hangul CC digraphs and 16 trigraphs, but there's no reason you couldn't create as many as you like: I mean, look at German dtsch
. You'd have to create new OpenType fonts for all those new syllabic blocks, but it's certainly workable. On the other hand, I have a suspicion that the Hangul Society may be interested in keeping the system as much like Modern Standard Korean as possible, which would definitely limit things in the ways you said. But still, hangul started off with more vowel letters and more consonant letters than Latin or Greek.
So all the rest of the 300 minority groups just use the Latin Alphabet for their languages?
Not all 300 of them are considered minorities, although there is no ethnic majority. Even the Javanese, the largest ethnic group, comprise less than 45% of Indonesia's population.
Although many of the ethnic languages, including Javanese, Sundanese, Batak, Buginese, Makassarese, & Balinese, were originally written with brahmic abugidas they all adopted Latin after Dutch colonization.
At least Javanese and Balinese are still written that way. Children still learn it in school, though they don't get much practice using it, even though it's still found all over the palaces and temples. Batak and several other languages still use their scripts as well, though in many cases they may be no more robust than Arabic is in Hausa and Turkish. But the choice is there: if, say, the Toba wanted to assert their cultural independence, they could move to switch from Latin to batak full time. And remember, Malay (and maybe Acehnese?) was once written in jawi. I'm not sure it's just pressure from the national culture for Latin, because for small cultures, a technologically established script like Latin just more practical: all the technology and infrastructure is already in place. You can't use batak for email, and it would be a pain to even get a newspaper published. But if the desire is there, they could get it done, and the tech is getting easier and more inclusive all the time.