HTML is the most popular language in the world. The core of HTML may have "stagnated" because of JS, but the alternative would probably have been extinction and replacement with something that didn't decline to include programmability.
I also don't think HTML has actually stagnated, though. HTML+CSS+JS is a solution, and it's changed a lot over the years. The HTML piece has experienced the smallest set of core changes, but that's because it already does what it needs to. Adding a modern file upload element might be nice, but it's not necessary. (Also the behavior of upload on the client side is really a browser choice. Browsers could make this a lot prettier without changing HTML at all.)
Like x86 assembly is, sure. Though way less well-suited to its modern purpose than that is. It's alive by accident and momentum, purely.
If it did what we needed it to do we wouldn't have burned who knows how many (tens of?) thousands of person-hours creating half-complete solutions to its various plain-as-day shortcomings. Input wrappers, table sorters, sequential image viewers, and so on. We did finally get video but it's still usually wrapped in custom UI driven by JS. It's not complete and featureful enough to exist as a good rich document format without JS (due to its long stagnation) but is also really far from being a good choice for laying out and describing applications (because it was never meant to be).
Agree that browser vendors are the ones who'd have to fix all this. The time for that passed a long time ago, of course. I just think it'd be nice to have a hypertext document network that doesn't have on-client spying as a first-class, built-in feature that you have to go way out of your way to even partially avoid. Creating Javascript and giving it such wide access was what killed any hope of that. I consider that the original sin of the modern web, that the platform itself is fundamentally and irreparably insecure (from the end user's perspective).
I also don't think HTML has actually stagnated, though. HTML+CSS+JS is a solution, and it's changed a lot over the years. The HTML piece has experienced the smallest set of core changes, but that's because it already does what it needs to. Adding a modern file upload element might be nice, but it's not necessary. (Also the behavior of upload on the client side is really a browser choice. Browsers could make this a lot prettier without changing HTML at all.)