"The point here is that in semantic web there're supposed to be lots and lots of different ontologies/schemas by design, often describing the same data."

Then that is just another reason it will fail. We already have islands of data. The problem with those islands of data is not that we don't have a unified expression of the data, the problem is the meaning is isolated. The lack of a single input format is little more than annoyance and the sort of thing that tends to resolve itself over time even without a centralized consortium, because that's the easy part.

Without agreement, there is no there there, and none of the promised virtues can manifest. If what you say is the semantic web is the semantic web (which certainly doesn't match what everyone else says it is), then it is failing because it doesn't solve the right problem, though that isn't surprising because it's not solvable.

If what you describe is the semantic web, the Semantic Web is "JSON", and as solved as it ever will be.

A "knowing wizard correcting the foolish mortals" pose would be a lot more plausible if the "semantic web" had more to show for its decades, actual accomplishments even remotely in line with the promises constantly being made.