> how many layers of abstractions built on top of other (redundant) layers of abstraction really make sense?

As many as needed. This is a political problem, not a technical one.

OSs don't want to provide one common framework for writing and distributing sandboxed one-click-install write-once-run-anywhere applications. So browsers are solving this problem on their own.

Maybe you don't care about this, but users, developers who need to write universal apps, and their marketing managers definitely do.