Not sure what you really mean. If you mean the loss of "View Source" then I think we lost that a long time ago for anything but the most trivial of pages.
I used to say that view source was why the web was successful, and maybe it did help at the start, but I think we sailed past the point where view source stopped being useful years ago. Everything now is minified, and sites are too complicated for anyone to pick out what's going on from the original source let alone the actual served source...
> How did you get into the Web? Was being able to easily inspect and change what was happening in apps you didn't have intentional source access for (web pages/apps) not key to that?
I'm a weird case, I got into the web by reading the standards. (Then I got annoyed the browsers didn't follow them, so I got into browser QA. Then I got annoyed that the standards weren't clear enough to allow test cases to be written, and I started editing the specs.)
> Have you gotten value from the ability to quickly use others' extensions of useragent behavior that seem highly unlikely to work with this approach? Or will they be able to?
I have not. A lot of people do like extensions though.
Other platforms have managed to get concepts like extensions, e.g. on Linux (well, X really) I can replace my window manager, on Windows I can install DLLs that add features to Windows Explorer, on Android the "share" button can share to a list of applications, etc. There's lots of examples.