My naive fix would be to disable extensions from accessing form field data without explicit approval. Hell, add different approval boxes for read, write, and hidden-text.

What am I missing?

Say you have an ad-blocker and you don't allow it to touch your forms. Five years later, the ads have moved all into form fields.

Never mind the technical challenge to allow doing anything with the DOM but disallow reading the forms. Like, prevent the forms leaking its text when you do funny things like testing character width via line breaking or font changes.