There are a number of places where PDF has advantages that HTML, etc can't match. For forms, the PDF spec has electronic signatures, which means that you can sign legal documents without having to find a fax machine, or mail things back and forth. For print documents, it can do things like multiple column text in a sensible manner, across page breaks. As a general document format, it allows you to actually describe paper sizes, etc, so you can email someone a document that looks like it will when it's finalized. For archival, it means you can, without a bunch of hacks, create searchable documents of old, scanned in content. So things like electronics datasheets are searchable, and still retain the formatting of the originals. PDF continues to exist because there are so very many places that the alternatives are an even bigger pain in the ass, fix those, and then you can start to get rid of pdf.

Would postscript classify as an "even bigger pain in the ass" with regards to pdf?

All sensible uses of PDF (except your first one; electronic signatures are ridiculous, but let's not get into that).

I'm talking about where pdf is used and shouldn't be.