[snazz]

Because the entire advantage of the web from the perspective of the end user (as opposed to desktop applications) is that you can expect it to work the same everywhere. If everything were separated, it wouldn't make a difference, because Chrome and MobileSafari will continue shipping one integrated stack, even if they're forced to open up APIs for modularity.

What tangible advantage would this have? Having competing browsers seems possible, but since end-users wouldn't be choosing individual browser components anyway, it wouldn't actually improve anything.