I think it's your latter point that's the key issue. This is something that has been able to be predicted from years ago. Most people do not use phones for many hugely intensive tasks: email, messaging, audio/visual (music, photos, videos), and a handful of generally low resource cost apps. Like you mention it's exactly what happened to PC. Phones that are now years old can perform these tasks practically just as well as new phones and will be able to do so for the foreseeable future, so why would you buy the new phone? The fact that new phones have become much more expensive is just going to accelerate a process that was already happening, rather than being a cause of that process in and of themselves.

And really this process has repeated in countless consumer hardware products besides PCs. Televisions were another example. The change from CRT to flat screen was a pretty big change. And the TV's kept getting bigger, flatter, and with better image quality. But eventually people decided that their TVs were big enough, flag enough, and looked good enough. And so TV sales have begun their extremely predictable plummet.