The iPhone 'slow down' was to keep the phone operating with old batteries. Without the throttling the phone would just shut down abruptly under certain conditions.
Part of the reason Apple has this issue at all is that they provide continued support for their hardware years after others have stopped even giving security updates.
Apple has also a multi-year lead on things like chip performance in their devices (and more subjectively hardware design, software performance too).
I used Android nexus phones for a few years before jumping into the Apple ecosystem with the iPhone 5 (I had used macs for a while prior to that since OS X 10.3 and played with a variety of linux distos alongside it).
If Apple stuff starts to degrade compared to competition it will be a hassle to jump to a different ecosystem. As it happens, over the years it's just gotten better further ahead of the competition (with some mistakes: butterfly keyboard, touchbar).
The premium and their authoritarian approach to vertical integration and good defaults isn't for everyone, but I think it's really great and worth the premium. Other companies seem to be all over the place without much of a unified vision of what they're doing. Google in particular is extremely bad at this.