I'm not thinking of it as "fixed" so much as written languages do most other potential jobs better/clearer/faster than emojis. It's not sufficiently complex/composable enough to pose as a real challenger for expression.

Any useful utility for emoji will be at best in addition to written language, not in replacement of it.

And the major blocker is definitely the lack of composability; you have a very strict and very limited set of options to work with (and without composition, the rule "limitation breeds creativity" doesn't apply), so I have little hope for significant creativity to appear