"The initial release date of the series has nothing to do with how a new game will run on current hardware.
Given current market and developer trends, it is not looking good for low end hardware."
An HD 5850 isn't really low end. It's upper low end at this point. Use passmark. DirectX 12 should shake things up quite a bit and I'm looking forward to it - currently I own around 9 gaming PCs, 5 of which I think are DX12 ready now.
I agree with your last statement, however, I disagree (at least in part) with your first. Let me clarify your lack of reading comprehension by saying that I never mentioned the release date of the series. I'll post the exact quote here again so you can re-read.
"Fallout was started before the HD 7000 series cards were even released, so it's likely they will run on an HD 5000 series card."
Software has to be written and tested using existing hardware. Modern cards were being crafted in hushed rooms. Therefore, one could surmise that Fallout 4 will likely at least run on cards a few years old - it's not likely that it will run at highest detail. Even Crysis could run on low end hardware, it just did so at unplayable frames.