If you want content to be able to be presented in different ways (such as a Reader Mode) that reflect the document structure, there are far better options than PDF. This is Adobe trying to extend their moat. Just Say No.
You may as well be saying that there are better options for producing readable text than HTML, because that's what Reader Mode is for. Yes, but entirely irrelevant. Reader Mode is for the reader to use, not for the writer to use. Liquid Mode is for the reader to use, not for the writer to use. See the parallel?
What the writer wants to present is structured layout based on design. Sometimes that's what the reader wants to see, other times not. That's why websites aren't all flat text and browsers have reader modes, not the other way around.
HTML is not known for providing print-ready layouts. This is just a different way of viewing documents that are designed to be print-ready. Whether they choose to implement the output of the conversion in HTML or some other method isn't relevant to the announcement.
What the writer wants to present is structured layout based on design. Sometimes that's what the reader wants to see, other times not. That's why websites aren't all flat text and browsers have reader modes, not the other way around.