I still use the S6 and I still feel like I wouldn't be getting my money's worth when upgrading. I was always the first one to get excited about the newest phones, but the newest phones of today just don't feel like new, exciting technology, if that makes sense. Now it feels more like getting a new model of a refrigerator while already having a modern one that works perfectly fine. I never thought I'd feel like this about a phone upgrade, and I have never used a single phone for even remotely as long as I do now.
They do exactly the same things that several generation old phones did, and not even much better, but they are much more expensive. The software updates are completely skippable, as they don't bring anything important.
Android Pie brings improved adaptive brightness as its headline, which is neat, but 5 years ago it wouldn't even be worth mentioning on a release note for a full version upgrade. Not to even mention that Pie doesn't do anything worthwhile under the hood, it actually seems to decrease the performance by a tiny bit and the battery life comes out as even with Oreo:
https://www.gsmarena.com/android_9_pie-review-1802p3.php
To put it into perspective, even if you're on a now 3-year-old Android release (Marshmallow), there were still no performance improvements and no major features that you would be missing out on, and now we know that they won't be there for at least another year. Heck, I bet that an everyday user likely wouldn't tell a difference between Marshmallow and Pie.
I miss the days when replacing your phone after 2 or 3 years felt like a whole new world, exciting new technology. The first Galaxy S was a groundbreaking phone at its time, but jumping from it to the S3 was absolutely insane in all possible regards, and that was merely a 2-year upgrade. Now I feel like replacing my phone only after it stops working, and I hope it doesn't happen anytime soon, because it'd feel like spending money without getting the improvement I should be getting after replacing a now 3-year-old phone. Again, it would feel just like replacing a modern refrigerator that stopped working - a burden rather than excitement, and I never felt that way before.
Is that cheap for a second hand s7?
I would say it's a fair "buddy-price". You could get more if you tried if it's in great condition, but I'd sell it for a similar price or maybe even a bit lower to a friend. You didn't lose any money on the deal.
