And even then, they replace batteries now on the cheap ($50 I think). I would say the biggest limiting factor would be OS updates; they guarantee updates for 4 years, which is the longest in the industry, but may be shorter than the lifetime of the device would otherwise be

I'd still be using my 6S+ if this hadn't happened to me. Apparently there's a software problem with the optical stabilization, and it goes haywire. A fix is to override the stabilization solenoids with a magnet next to the lens. Not happy that Apple never fixed this bug they introduced.

The other thing that went wrong, also apparently in an OS update, is that TouchID won't work anymore. The hardware isn't recognized.

My front camera is doing the same thing!

I looked into it a bit, and found that this happens when a piece of dust gets into the lens, which steals the focusing of the camera, so it shakes because it oscillates focus from the field of view and the piece of dust. Sadly the camera module needs to be replaced, which can be purchased cheaply but medium tedious difficulty to replace - or pay to have it done not as cheaply.

That depends a lot on what's meant by "refurb". I've had some that had had a screen and battery replacement, and they work and last like new, as they should. I've also had some that were sold as "refurbished" despite having had, at most, a wipe down with Windex to get the fingerprints off. Those don't do so well, but if you're handy, you can do your own refurbs with $80 in parts and an hour or so of work, and get the same like-new result modulo whatever damage exists to the case.

I should’ve, but I was both slow in realizing the problem and lazy in fixing it.