I think there is a limit on what you can add that keeps it worthwhile upgrading.
Just recently lost my z3 and need to replace it, and looking at the z3+ and z5 is it worth paying the extra? The z3+ seems like such a step backwards its actually cheaper to buy in a lot of places than the older z3, the z5 really whats it adding? Nothing special enough to justify paying £300 more. I liked my Z3 for the battery life, as long as the camera takes a half way decent pic it'll do. Everything else seems an incremental increase I'm not really convinced the fingerprint scanner would have defeated the person who lifted my phone. I'm not actually sure theres much i have seen that would make me go from the z3 at the moment tbh.*
So what am I paying for convince me phone company that the several hundred quid phone is obsolete 6-12 months later, and that I should pay a load of cash out again, I do not even
upgrade my pc that often its uneccessary, and I use that for things like games.
*ok if someone wants to send me the Motorola X-force to long term test I am cool with that (no really drop me a line), I do like that mostly because I tend to treat my tech badly though so unbreakable seems like an actual improvement, versus ooh it now photorgaphs with 2 extra MP on the camera.