If you want to always run the latest and greatest version of Android chances are you will be able to run it if you know how to flash a Rom. Even when devices are end of lifed by the OEMs rom developers keep cranking out updates sometimes even years after. Some rom developers are able to release their own builds even faster than Google. We saw this with the Nexus lineup when Lollipop was first released. Several developers had their own builds of Lollipop released for the Nexus devices days before the official builds were rolled out to older Nexus devices.
The 5.0.2 Pure AOSP rom by "gchild320" brings you a pure unenhanced build of android open source project to your Nexus 9. This is not rooted and includes no performance enhancements. Its just pure Android! You will need to have an unlocked bootloader and TWRP installed. You will also need to grab the Gapps to flash separately. Head to the link below to download the rom.
via XDA