By combining this with an extended form of SRI that supports hashes signed by a private key, it would be possible to bring the security model of web apps almost

By combining this with an extended form of SRI that supports hashes signed by a private key, it would be possible to bring the security model of web apps almost up to the level of desktop apps. You might still have to Trust On First Use whatever key/identity was signing a given version of a web app (at least the client-side component of it) but a browser could ask you "Do you want to update to version X.Y of this web app?" before running any JavaScript that you might want to check the release notes or reviews for first. Ideally this would be combined with something like Binary Transparency, where the new version has to have appeared in a public log for some time, and with no trusted third parties publishing a "Do not trust version X.Y" warning in another public log, acting as a sort of distributed immune system for the web.