It's a chicken-and-egg problem. When most people write in "phonetic" script, people stop using ambiguous words that sound alike, so these words fade away, which further strengthens the adoption of phonetic script.

If everyone around you uses Chinese characters, there's little incentive to avoid homophones, so they will remain a part of literary language, reinforcing the use of Chinese characters.

That is exactly what I meant. The theory that w/o Chinese, makes languages like Korean/Vietnamese 'worse', is a myth constructed by some sino-centrists. The decision to keep or deprecate Chinese characters is political, but people find their way to adapt and evolve with the new language.

Very true.

And indeed, Korean words that come from Chinese (about half) have a much higher ratio of homophones than in the native Korean part.