Has anybody else observed this and when did the trend start?
Apple points to Intel and says they haven't released many huge changes in awhile, and that is true but Apple should have been lowering prices over time.
Apple probably thinks they have most users covered. Since the cheese grater, there has been a huge shift to laptops. If a user really wants a desktop there is the mini, iMac, iMacPro, and now MacPro. The MP is so far out in the pro realm though that Apple may have inadvertently opened a gap in their lineup between the iMacPro and MP. If they do something to fill that gap remains to be seen.
IMacs are nice-looking terminals and playback-media stations, that’s your entry-level. If you want real power, you get a MBP. Only if you want huge power that the MBP really cannot deliver, you get a MP.