Windows 8 and Windows RT, for example, are in no meaningful sense "the same OS", they run different software bases. They share some common code, but so do Linux and Android and OS X and iOS, and to try to call those the same OS would be both confusing and inaccurate. iTunes is flawed for the same reasons that Windows is, they were intended to do a few simple things and have had every function on earth grafted onto it.
In both cases the right approach would have been to blow it up and start over from scratch. Apple switching to OS X was an example of this, The new core was small and fast, and old software was run by virtualization for a few OS revisions.