I'd argue that having a sandbox that can run binaries with a limited and customizable feature set is actually a good thing for the web. I think there are more technically competent solutions, but the fact that WASM is available on virtually every machine and os, makes it pretty powerful.
If implemented right WASM might speed up our web apps, keep the browser sandbox that is actually quite nice, and run on pretty much any machine. If they open sourced the code, that'd be even better.
Between minified js and WASM, I think I'd take WASM (I can't understand minified js anyway). Between a pure html site and WASM, I think I'd take the pure html site (but I don't think we will be living in that world anytime soon).
TL;DR, from what I can tell: Dropbox was using a JS bundler that didn't support code-splitting or tree-shaking (y'know, the staples of modern JS bundling) and swapped to one that does. Not that there aren't plenty sub-optimal components in code I work on, at home and at work, but there's nothing revolutionary going on here.