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.)
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.)