Supporting that makes sense to simplify the use case so that people don't have to write JavaScript for simple changes, but that doesn't imply they'll replace the JavaScript API with a purely declarative API there.
The difference between that and the traffic modification API is the threat model. The third party were to compromise uBlock Origin's servers, they would have a frightening amount of power over millions of machines because uBlock Origin essentially self-modifies; it downloads new rules for what should be blocked. So that breaks the security guarantees Chrome wants to provide; they can't say that the lock icon on a website means anything if a Chrome extension is allowed to arbitrarily modify traffic back and forth as a man in the middle on the last mile and Chrome web store maintainers haven't looked at the source code it's running.
I don't think there's a similar security threat for cosmetic changes.
