Personally, I think 8 GB of memory on a MBPr is anemic at best. Knowing you can never -ever- upgrade it makes 16 GB a no-brainer. The OS, apps and files are only going to get larger...
There is a huge flaw in your entire line of logic. It assumes that the computer purchased today persists, in perpetuity, while the buyer's needs grow at a fast clip. That is not a real world constraint.
The market for used Macs has been, is, and will continue to be strong. Moreover, many people in this market do not have intense needs. The option that your line of reasoning overlooks is purchasing a computer that meets the needs for the immediate foreseeable future, and then
if those needs grow, the person in question can sell the used model and get a new one.
Furthermore, if those needs really do end up growing, then it is likely that the user in question will desire much more than just another 8GB of RAM down the road. Faster processors, GPUs, SSDs, and who knows what other goodies will also likely have appeal.
A lot of people get obsessed with the idea of "future-proofing" their purchases. Having owned about a dozen different PBs and MBPs over the last ten years, I contend that approach is often misguided and occasionally silly. Base models of these laptops are almost without exception the best bang-for-your-buck. Higher end configurations depreciate at significantly higher rates. If you don't mind the hassle of trading up models, it can deliver huge benefits in addition to being extremely cost effective.
