To put it into perspective, that's larger than the entire post-install size of the original DOOM PC video game, including all sound and artwork.

Not really - imagine if you're looking to create a fully featured video editor, complete with 3d effects and whatnot. It could well exceed 15mb to deliver all the required functionality.

Shouldn't this be a native desktop application versus foisting it onto web clients? If we're talking "fully featured"?

This is where the paradigm shift is coming. Your past experiences say it must be a desktop native app, but I believe things like WASM, web workers etc will allow such things to run comfortably in the browser.

I really do believe people will build for the browser more than they will for the desktop because it's a more universal platform.

I can even imagine a future where, for the majority, the only native app you have on your machine is a browser, and everything else is some variant of a PWA. I really feel this will happen very soon.

Why? I can't wait for this world where everything can run on Linux.