It must be really hard to maintain and add features to such a huge old application. Platforms have changed underneath it, the devices and services it connects to have come and gone, Objective C has changed a lot as well. And then there's the Windows version...

Still, iTunes is crap. I hated it so much that I wrote my own music player for iOS, just to avoid using iTunes for putting music on my phone [1].

My guess is that sooner or later, iTunes will be dismantled and the features will be split into multiple smaller apps. There will be a music app that JUST plays music from the Apple Music account, a podcast app that JUST plays podcasts, a sync app that JUST helps you manage stuff on your phone etc. Some features will be removed (Internet radio, ripping & burning CDs). And of those that remain, only a few will need to keep a Windows counterpart. That way, things will be easier to maintain.

[1] Tiny Player: https://itunes.apple.com/app/id1140849233