My point was that — allowing that "the web" is just arbitrary 'stuff' at URLs, and that a "web browser" is a tool for viewing that arbitrary 'stuff' — there are always going to be novel non-standardized, proprietary document types residing at URLs, that people will nevertheless expect their web browser to display for them (and rightly so!)

How will users view these documents in a timely manner (i.e. not on the W3C's glacial schedule)? Whatever your answer, it's going to necessarily involve some kind of arbitrary code execution at some point. Whether that means Javascript; Java/ActiveX applets, or PPAPI (NaCl); Chrome Apps or other in-browser centrally-managed-lifecycle apps; installable NPAPI plugins like Flash; or COM component servers (as seen when e.g. IE/Edge is navigated to an .xlsx file URL.)

Presuming the user discovers they can't view document X, and therefore goes and hunts down tech Y and installs it to make X viewable, all such "tech Y" are equivalent in the sense that they're someone's arbitrary proprietary code, that the user is rolling the dice on hopefully allowing them to view X, while not doing anything other than allowing them to view X.