Hi Nick - In talking to customer support yesterday, I was told that ArcGIS for Windows Mobile for 10.3 is due for release in late January. My guess is that I don't think ESRI is trying to force everyone to collector, rather the Windows Mobile team probably didn't know or otherwise couldn't update Mobile until desktop was out - but again I'm just guessing.
In the meantime, after upgrading my desktop to 10.3 I discovered that the CreateMobileCache_mobile desktop tool from the 10.2 mobile install does still function and produces a valid cache. However, the latest python 32-bit idle (2.7.8) will not run the command. I have not tested the SyncronizeMobileCache tool.
I have not tested anything yet with the Mobile Content Server or Mobile Project Center but my guess is they will be unaffected.
Word to the wise- do not re-install or attempt a repair of the mobile 10.2 install after installing desktop 10.3 - you will lose the desktop tool functionality-
David Coley
Sarasota County GIS