I don't think a lack of space is the only or even the major problem with the slow adaption rate.
iOS 8 is simply too resource-intensive. I've installed it on my iPhone 4S and ever since my phone is painfully slow. Apps take a lot longer to load, and it takes longer to switch between them. Performance within apps has suffered, too. Everybody I know with an iPhone 4S or 5C has similar problems.
As a consequence I discouraged all my friends from updating unless they have an iPhone 5S. Having a faster and more responsive phone is clearly worth more than the few benefits you get from the update.
I assume there are a lot of people out there with a slow phone after the update discouraging others from updating.
For iOS 9, Apple should try to make the operating system less resource-intensive, not more.