LibreOffice does let you create editable PDFs. They do this with a very elegant solution, they embed the Open document format within the PDF. This takes very little additional space, because the Open document format is compressed. I think the LibreOffice Solution is quite elegant; Open document format is already a standard, we don't need to create another one. And most important, it works today, right now.

It would be a lot of effort to create a document format with the kind of richness that PDF supports. I am dubious it would be worth it.

I think most people do not need an editable PDF in the first place, so this is a minority problem. If you do want this, for most people there is already a working solution... just store Open document format within PDFs.