When Apple puts features that cause old devices to be slow because they have difficulty supporting it: OMG they should have released the update without those features for old devices! PLANNED OBSOLESCENCE!!!
When Apple releases an update but some features are not on older devices: OMG those devices could run those features! PLANNED OBSOLESCENCE!!!
When an update makes an older device run badly: OMG it was fine on last OS, they should have retired it instead of giving it one update too much! PLANNED OBSOLESCENCE!!!
When Apple retires an older device while it still works fine with its latest supported OS: OMG it's still working fine on its latest supported OS version, they could have given it another year! PLANNED OBSOLESCENCE!!!
Apple releases update full of features that bring lots of bugs: OMG they don't optimize on purpose! PLANNED OBSOLESCENCE!!!
Apple releases update based on performance and optimization: OMG now they optimize to repair all the bugs of last OS! They clearly do PLANNED OBSOLESCENCE!!!
I could go on and on with opposite scenarios that somehow all lead to planned obsolescence accusations. Anything Apple does has become linked to planned obsolescence, just like the "Apple is dying" thing. If they release a buggy features update, they are dying, but if they release a performance update, they are also dying because there's not enough new features. I'm not saying Apple is perfect, they are not. For all we know, there might really be planned obsolescence, but we have no proof of that. A device slowing down is not proof of that. It doesn't mean with certitude that they slowed it down on purpose. iOS 12 being faster is not proof of that. Bugs are inevitable in an OS, and with iOS 11 and its numerous big problems throughout the year, Apple probably realized that they needed to optimize it better this year. Just like any good company that releases a buggy OS would do. I think the reason the planned obsolescence accusations are not taken seriously anymore and are easily dismissed is because every scenario possible has been turned to mean planned obsolescence.
Also, on the subject of Apple Watch S0, when WatchOS 5 comes out, it will have been 3.5 years since it came out, which is pretty much what I expected its lifetime would be considering it was already pretty slow at that time and battery barely lasted a day. Also, why would you consider upgrading a watch that you still think works fine for what you use it for just because it can't have the latest OS, if you are also not interested in newer models and OS features?
Each and every scenario you mentioned can be resolved with downgrade support. An option supported by Apple’s competitors.
I do admit I have zero knowledge on development and all but leaving a device on an OS which is slow and forcing the owner to stay on it and tolerate the slowdown is planned obsolescence. Release every damn version for 5 years and give the customer the option to switch.
Result- Everyone is happy on whichever version they like. At the moment my Pro is still on iOS 10 as going to 12 is like venturing into the unknown and if I don’t like it, I am SOL.
Apple certainly has the resources to provide security fixes for multiple ios versions as Microsoft and Google do.