There are a lot of 8GB devices that could be upgraded to iOS8 but are unable to because of lack of disk space.
http://store.apple.com/us/buy-iphone/iphone5c/iphone-5c-8gb-...
And by disk space I mean after the user has installed the typical number of apps, photos, music etc. My partner is an example of this. She has to choose between deleting apps and doing OS updates.