I think that's really the key to this that makes the "worse is better" argument miss the mark here. HTML succeeded because anyone could open up a text editor, learn a few simple rules, and have a web page in short order. It's fantastically more complicated now, but unlikely to be supplanted because we have two and a half decades of HTML+CSS+JS out in the wild. People work hard on cross-browser compatibility because it's not going away, not because it's fun.
And all that fragile, brittle content out there will eventually break. The question is when compatability is lost, and in the name of what.
Keep in mind that I'm specifically targeting text and textually-oriented document content. The modern Web can be considered generally as having four principle modes, three ofwhich I'd treat separately: documents, as described, commerce (probably hived into a dedicated application), media (likewise), and apps (which want a VM engine, e.g., Chromium).
A surprisingly large set of apps, and certainly many significant ones, are principally document-and-discussion engines, for which lack of an intrinsic model within the document markup and client presentation is the raison d'etre of those apps. Either having a paired discussion platform, or integrating discussion into the browser itself, would address much of this.
Other content elements which have become significant online include both advertising and DRM. These have been mistakes.
There are numerous "HTML page simplifiers" (most based on Readability's engine AFAIU), which might shim behaviour and compatibility for legacy pages.
And content itself is text, not code. Slavish backwards compatibility is not a strict requirement.