Fixing that is not too hard. If you look at Mac OS X Lion, for instance, part of that already is in place (in a different domain). If you download a .xyz file and double-click it, the Finder will suggest looking for applications that can open .xyz files.
I think a logical extension would be that the system would automatically download free viewer apps instead. Add a seamless way to upgrade to the editor app, and you are all set (tangentially related: I think a thing like OpenDoc using that infrastructure could succeed in today's market)
Could they download and install these viewer apps in a fraction of a second or two, because thats what a browser does. The web works on the basis of progressive enhancement so all the browser needs to fetch is one Html file which it can display to the user while it downloads all other resources. I don't see a native code matching that.
"all the browser needs to fetch is one Html file which it can display to the user while it downloads all other resources."
If only. There are pages where that is possible, but typically, at the very least, browsers need to download some css, too (for example, google.com, on my system, does 9 http requests of which one fails). Even with inline css, a browser can discover, halfway through a file, that some css moves a div up top, floats it, etc.
Add to that 'not everybody' specifies image dimensions in the HTML and that Javascript that runs on page load may do DOM updates, and you get a much more complex model. Deciding when to start rendering a page is somewhat of a black art. If you wait until you are sure you have all information you need, the browser will appear slow; if you do not, you will have to restart page layout sometimes.
"in a fraction of a second or two"
Pages that load at that speed are rare. Again taking google.com as example, it loads almost 200kB here (most of it from cache, but that would not be much different in what I envision). When running from cache, rendering that simple page takes about half a second, I guess (FireBug reports 700ms and up, but it will have some overhead)
I think a logical extension would be that the system would automatically download free viewer apps instead. Add a seamless way to upgrade to the editor app, and you are all set (tangentially related: I think a thing like OpenDoc using that infrastructure could succeed in today's market)