Well fix the things that:
a) Makes me mad at Apple
b) Makes me NOT buy a new iPhone despite wanting to.
c) Makes me determined to ditch my current iPhone when it stops working
I am currently voluntarily stuck in refusing to upgrade and trying to circumvent Apples attempts at forced obsolescence. I have a Mac with Snow Leopard (Mac OS 10.6) and an iPhone with iOS 8. If I upgrade the Mac, many of my old programs stop working. If I buy a new iPhone, my Mac wont talk to it. I have been in the same situation before with the advent of MacOS 10. It took me a long time to make the jump and it was a hassle, I had to upgrade everything and spent days if not weeks trying to get stuff working as good as I had it with MacOS 8/9. When I wrote similar posts in forums about this, people were unsympathetic and pretty much said "just buy a new Mac". Well, then I had no money. Now I have no time to spend installing a new OS even if I could get my old programs running using MacOS X server or something.
I will make the jump when I must and when I have time.
But I would be a lot more happy if I could just buy an iPhone with a headphone jack that would sync with my current Mac. An iPhone that did not nag me all the time that I should upgrade.
Therefore
a) Sign all versions of iOs when people wants to install. People should always be able to downgrade, or even just install an old iOS to temporarily run an old program. Personally I don't know if I can do a full restore of my iPhone with it's "old" iOS, and because of the cryptic messages in iTunes I have not dared to try.
b) Stop nagging about new os upgrades, a gentle reminder works nicely, getting a message roughly every day is just obnoxious.
c) Absolutely stop downloading 1 GB iOS upgrades into my iPhone without my permission. There is no excuse for this.
d) Give us back the headphone jack on some iPhones. Check who would be willing to pay for this. If you can prove sales are bad fine, but when I commute, I see those white headphone cables everywhere. Wonder what would happen if everyone switched to Bluetooth.
e) Better backward compatibility.
For instance allow old versions of iTunes to sync with newer iPhones. I don't expect any new functionality, but Apple has just blanket stopped the functionality, when they could keep the same functionality on older product. With that I mean that iTunes could sync the same data, plus a requested amount of data with unknown content. That people who upgraded to iOS 9 on the iPhone could still synch with iTunes 11.4, while people who upgraded with their iPhones connected to iTunes could not synch proves that this was a choice on Apple's part.
f) Improve your products. Theres a joke video that the 2016 Macbook Pro is just an 2015 MacBook Pro with ports and function-keys ripped out.
g) Think "What would our customers want?" and "What would they be willing to pay for?", not "How can we force our customer base to buy new products.". If you chase away loyal customers, they are not coming back.