Except that around 90% or more of the time I find myself having to open PDFs using a real native PDF reader because pdf.js is too slow, or it improperly renders the PDF, or it won't allow me to fill out forms the way I want, or some other issue like that.

So even with pdf.js present, I end up using a proprietary reader most of the time. The added hassle or reopening PDFs in a separate reader most of the time exceeds the supposed convenience of the reader being built in.