You put too much of a focus on developers. Most people don't care about development within the browser, they care about development within web sites. The philosophy that the browser ought to merely be a window to other sites with as little chrome as possible has been advocated by Chrome's (and Safari's with the advent of Snow Leopard) multithreaded processes. To date, I have still seen no good reason to use extensions, and the majority of people I discuss web browsing with agree. (The biggest sticking points are Greasemonkey and Adblocker, and they're very minority points, particularly when projects like GlimmerBlocker work cross-browser and allow both extensibility and blocking.)