IDC have new research showing that tablet growth is slowing significantly. It also shows Android is expected to have 67.7% market share this year and iOS 27.5% market share. IDC explain that the decrease in tablet shipment growth is being caused by people holding on to their devices. Ironically, this is due to OEMs updating the OS to keep it fresh, especially on iOS.
I also suspect tablets aren’t replaced as often as smartphones because they are not tied to carrier contracts. The carriers are good at promoting replacement at the end of contracts. Also, they tend to not have as ‘wild’ a life as phones and are less likely to to need replacement due to being damaged or stolen.