If WordPress outputs semantic output that instantly gives you a lot more than 0.0001%. The rest would follow as soon as it improves discoverability of their content

Wordpress can't magically infer semantic meaning from user input any better than Google can. The whole point of the semantic web is to have humans specifically mark their intention. A better UI for semantic tagging would help for that, but it would still be reliant on the user clicking the right buttons rather than just using whichever thing results in the correct visual appearance.

> 0.0001% of content authors would employ semantic markup.

You don't think we'd have rich tooling to support it and make it easy to author?

Once people are using it with success, others will follow.

The breakthrough would be when Google were to rank pages with proper semantic markup higher. Just look at AMP.

(Of course that won't ever happen, but that's what would be needed.)