...it will be interesting to see what percentage of its user base Apple disenfranchises when it does go.
I'm not sure how you'll find that out, other than anecdotally.
Well,
NETMARKETSHARE reports and others of its ilk present themselves as accurate, rather than anecdotal, and appear to be accepted as such.
I poked around a bit on netmarketshare.com, and found their stats on OS usage for
Nov 2013 and
all of 2013.
Edit:
...some will keep on keeping on until their current hardware breaks, not needing the benefits provided by newer hardware and the software it requires/makes possible.
Yep, that's me. (Not having to worry about syncing iOS devices is a big saver for me and, I imagine, many others.)