Considering Apple intentionally slows older phones with firmware updates post new phone release, I think your argument that they're trying to protect the experience doesn't hold much weight.
Source, please?
I'm sure there are examples with newer devices and iOS versions, but I wouldn't tell, because I didn't get any other afterwards.
I’m sure they do it all the time, but this one incident actually caused worldwide news-coverage, even in tabloids read by normal people (that is, not just techies).
That’s the phone slowing the CPU when the battery gets old to prevent unexpected shutdowns. If you think this is “planned obselence” you misunderstand the issue.
iOS 11.3 now lets you disable this, so you can have full performance at the cost of unexpected shutdowns. Or you can just get an official battery replacement for $29 and your phone will be like new.