You Won't Need to Upgrade Anytime Soon
Android Oreo might be the last major Android update for the Galaxy S7 and Galaxy S7 Edge, but that doesn't mean Samsung and its carrier partners will stop supporting the devices in 2018.
Samsung typically cuts off major upgrades at the two year mark. This means the Galaxy S7 and Galaxy S7 Edge are in danger of missing out on an official upgrade to Android P/Android 9.0.
That might be the case, but the Galaxy S7 and Galaxy S7 Edge should continue to get bug fixes and security patches long after the major upgrades stop.
If you root your Galaxy S7 you should also be able to take advantage of features from the major upgrades you miss.
In 2017, developers brought the Galaxy S8's Bixby voice assistant over to the Galaxy S7. We expect that kind of unofficial support to last for several more years at least.
If you don't want to spend a lot of money but want a device that's going to keep, consider buying the Galaxy S7 or Galaxy S7 Edge in 2018.