Natural Evolution Indeed
I used to build my own PCs from parts. I got sick of dealing with the Windows ecosystem and being part-time tech support, so I switched to the Mac ecosystem to get a computer that just works.
Even so, on my current MBP (late 2008) I have upgraded the RAM from 4GB to 8GB, and the disk from the original 320BB (I think) to 512GB. Those were easy. I am a gamer, so I appreciate the dedicated graphics. I sometimes run multiple VMs at the same time, so I need the RAM.
So, you'd think I would be the bullseye on the demographic target for the non-Retina MBP. However, a few things have changed to make that no longer the case. First, hardware requirements simply aren't advancing as fast as they used to. The games coming out this year will run on 3-year-old hardware. I no longer try to keep up with expanding disk space requirements by upgrading my laptop; I store lots of stuff on my NAS drive instead and only tote around what I need to have locally.
The only thing I might want to upgrade is the RAM, maybe in 3-4 years. It's hard to tell whether that will happen or not. If I order a Retina MBP it'll have the full 16GB for sure.
So, the benefits of upgradability have decreased over the past 3 or so years. But, the benefits that accrue from the non-upgradability (lighter weight and smaller size) are ones I crave. Some laptops never leave the house, but I put on over 100,000 miles of travel per year. I have been saying for 2+ years that a 15" MacBook Air is my dream machine, but only if they would include dedicated graphics. And then they went and did it.
My bet is that the cohort of power users who really need upgradability is a subset of those who think they want it, and that the latter is a relatively small minority of the potential Mac base. For those people, the non-Retina MBP is still here, but I bet this will be the last iteration thereof. Maybe a speed bump in the winter, but after that we'll be looking at these things the same way we do iPads and iPhones.