Should I keep my iPhone 6s Plus?

I mean nothing is wrong with it and considering I drop it like crazy (case kept) it's actually pretty durable not even a dent or crack! Anyway it's almost 2 years old now and I'm trying to decide if I should trade for a Samsung galaxy s8 or wait until the new iPhone comes out? I'm skipping the 7th generation as they are almost identical

No

If it works fine & you don't need to upgrade, then don't.

The iPhone 7 (despite the name) is actually part of the 10th generation of iPhones by the way.

It's up to you. If you prefer iPhones than I would still use your old one until the new iPhone comes out.

Just keep it until it's considered old