I vaguely remember reading that they already had a post apocalypse project in early dev before getting the Fallout IP so I can see some measure if that 20 years thing being true as well. The world would certainly make a lot more sense were that the case. I do think Bethesda kind of tried (emphasis on TRIED) with Fallout 3 to write sensible reasons for traditional Fallout iconography to reappear so maybe they cared a little more back then than they do now. Fuck, maybe they just wanted to put the Harold quest in so much they shifted the timeline forward. Anyway, I guess it's not too important now. It seems like they learned all the wrong lessons from 3's critique with 4, including with the world space.