Also, we're getting into the differences between upgrades (what OP said) and updates (the thing necessary to keep up with security) here. I'm having a really hard time coming up with an OS feature introduced in a full OS upgrade within the last 10 years that I actually want. I would have been perfectly content with only getting security updates since.
- Live Photos. I won't consider any device that doesn't have something analogous, now. They're magical. Non-kid-havers may feel otherwise, but making every photo I snap of my kids a "Live Photo" is one of the most important improvements any technology thingy has provided in my entire life. (I think these are under the 10-year line? Very close, if not)
- Transparent OCR and text selection in images. The first time I used an image for a couple full minutes, copying text out of it, before realizing only when I went to share it with someone that it wasn't a PDF but a PNG, was when I knew this had to be a table-stakes your-OS-is-incomplete-without-it feature for me. Sure it could just be a feature of some program I've got installed, but having it in any native image view is way better than having to open some specific program to use it, a thing I could long have done, but never did. Making it first-class, fast, and transparent to the user, makes all the difference. I rely on it all the time now.
A couple off the top of my head.
And really, that's how it should be. Both of those things are not things the operating system does or should do.
Headphones automatically connecting to any of my devices that play audio, even switching between them.
And sure, a cryptographic coprocessor will make accessing encrypted data a lot more snappy. But decrypting that data in software instead will work just as well, albeit slower. Which really doesn't matter for 90%+ of phone users.
[EDIT] FWIW I'm in the so-called "Oregon Trail" mini-generation, so have definitely lived in and remember a world far more low-tech and connected than the current one, was a very late smartphone adopter, and have only reluctantly and recently come to admit that my phone is basically the only really useful computer I own for my personal life, and the "real" computers are mostly just toys for me that, at best, can replace only part of my phone's useful-in-my-ordinary-life functionality, in its absence. A high-quality internet-connected sensor and I/O suite in a pocket-sized package is just too damn useful, I've found.
