This is a big issue. It's one thing for a device to get slower over time as programs become more and more demanding. It's another to have a device that's running great and then suddenly slows to a crawl the moment after an irreversible OS update (what happened to my MacBook after Snow Leopard).
Apple uses dark patterns to get you to update and then blocks the downgrade path. __That__ is the part that's infuriating.
When Apple decides that the support for your iOS device ends, you have no choice but to upgrade, or risk running outdated vulnerable software.
The laptops, desktops are a different story since you can breathe new life to them by installing a free operating system.
The only realistic options for a smartphone are android and iOS. Between the two (and I have used both), I much prefer iOS.
Not for features or smoothness, mind you. I actually felt like my Android experience was better.
With google, however, I always felt like they were trying to sell me something and harvest my data through android. Not the case with Apple.
If a consumer buys something, there should always be a way to return it to the state it was at the time of purchase.