MicrosoftVery disappointing that this is the case (but admittedly not unexpected). As others have said, OneNote UWP is just not ready yet to replace its far superior predecessor.
In addition to some of the stuff already noted (though I am glad page templates and better tagging is apparently coming to OneNote UWP), I'd miss out on so many features. Audio recordings turn to a 'dumb' generic embedded file instead of linking to what I'm writing at the time. Linked Notes as a feature is entirely gone. Meeting details that only work for Office 365 accounts and not on-premises Exchange (how about just using the W10 calendar APIs?). Video recordings gone. Embedded content from other apps gone. I mean heck, I would say even the fullscreen UI is far better in 2016 for quickly switching between pages. And in all honesty, searching inked notes via a text entry has never worked as well for me in UWP as it does in 2016. (In fact, the only time it works at all for me is if I sync from OneNote UWP to OneNote 2016, let OneNote 2016 index it, and then sync those changes back to OneNote UWP.) I have no desire to convert handwritten notes, but one of OneNote's best features is searching those... and it just has never worked right in the UWP version for me.
This is like if you discontinued Word 2016 to replace it with Word Mobile (UWP). The only reason it's not causing a bigger fuss is that nobody knows how great OneNote 2016 is.
