The iOS 8 adoption rate is lagging behind iOS 7. From 9to5mac:

iOS 8 gained only 1 point from 46% to 47% since September 21st which should have well exceeded 1% gain by the millions of new iPhone 6/Plus shipments alone – even if not one person had updated their iPhone 5/s/c. Even more confusing is that the “Earlier” category of iOS 6 and before devices actually grew in percentage from 5% to 6% over the previous two week period.

I’m not sure if this is such a big deal just yet. There are a lot of 16 GB phones out there, and they tend to be pretty jampacked with apps, photos, and movies. That makes the iOS 8 update tough to install, since it’s requiring some 6 GB, at least in some cases. Still, if developer feels they can’t rely on iOS 8 growth similar to that of previous versions, then they won’t dare to rely on iOS 8 features exclusively. That’d be a shame, because iOS 8 is an under the hood release and it’s in all our best interest to see it widespread.