Part of the reason a lot of users are running the latest OS is that Apple sells twice as many devices each successive generation and the last three generations

Part of the reason a lot of users are running the latest OS is that Apple sells twice as many devices each successive generation and the last three generations (including the current) run the latest OS. Getting the last few percent of users to upgrade (assuming their devices support it) will be even easier now that iOS has PC-free software updates.