Dont worry, Apple will force the upgrade the same way they always have. Make the next version of iOS so freaking slow on older devices that its unbearable to use. People that stay on iOS[789] will eventually be forced to upgrade in order to get their necessary app updates.
My poor iPad 2 is so slow on iOS9 that its unbearable to use for any length of time. A5's are unbearable, A6's are not far behind.
It's not just Apple - it's the developers who release updates to their apps which, because they have the latest gear, are optimised for the latest hardware. I have an iPad2 and only recently upgraded it to iOS9 after 6? (I think, the one where they introduced the wobbly effect) - it got slow before I upgraded it because I updated all the software on it incrementally. It started lagging before I upgraded, indeed it was the reason why I upgraded the operating system. It didn't make any difference.
My poor iPad 2 is so slow on iOS9 that its unbearable to use for any length of time. A5's are unbearable, A6's are not far behind.