As e.g. the fine men and women of Int. J. POC||GTFO have demonstrated, it's possible to embed all sorts of weird and wonderful things in a PDF. I think a format where it's possible to include as much as possible is better.
Just look at HTML. Would you rather have it be impossible to extend beyond text, images and stylesheets?
I would. The modern web is a rich-get-richer hellscape of surveillance and advertising.
Form filling is part of the standard, specifically, the process of applying an XFDF to a PDF, which even allows you to do things like present a remote web form and then inject the data into the PDF.
html can include acrobat only JS and embedded flash and form filling crap.