And there is the fact that people/orgs lie/maintain poorly and thus you cannot trust any open tag/schema anymore than the data it is supposed to describe. You end up in a conundrum where your data may be sketchy but on top of that the schema/tags may be even worse, at the end of the day if you got to trust one thing it's got to be the data that is readily visible by everyone because it's more likely to be accurate and that's actually what's relevant.
To top all of that, tagging your stuff with RDFas just makes it easier for google and others to parse your stuff and then exploit the data/information without ever sending anyone to your site. If you are Wikipedia, it's mostly fine, but almost anyone else would benefit from receiving the traffic for a chance at value conversion/transaction.
All those metadata things are really idealized academic endeavor; they may make sense for a few noncommercial stuffs but the reality is that most website on the web need to find a way to pay the bills, making it easier for others to exploit your work is largely self-defeating.
And yes, LLMs didn't even need that to vacuum most of the web and still derive meaning, so at the end of the day it's mostly a waste of time...