Security updates are becoming more dire, but feature updates between versions has been dwindling since the early days. I've had an Android phone continuously since 2011, and while I could detail a laundry list of stuff we got from Gingerbread -> Ice Cream Sandwich or KitKat -> Lollipop, I can't think of many "game changers" from Marshmallow on that made each new update a "must have". Just trying to think of the ones I actually noticed or used:

Nougat: Slightly better battery with doze mode; Built-in Night Light so I no longer need cf.lumen or Twilight

Oreo: A better AutoFill API for my password manager

Pie: Adaptive battery? I lost the ability to record calls when my phone got the Pie update, so I wouldn't really consider that a feature.

So basically, other than having to use a third-party screen temperature app and needing to manually copy-paste passwords, there wasn't much meaningful difference in the experience between my Nexus 7 running Marshmallow and my S8+ phone with the Pie update.