Okay, here's my perspective take on this. If anyone feels there are any incorrect parts, feel free to correct me. For the sake of this message, let's call the majority of the Bethesda fanbase "Group B", and the pro-Obsidian, old Fallout enthusiast that makes up most of NMA "Group A".
______________________________________________________________________________________________________________________
It's out of pure coincidence (I think) that at first, the better Fallouts just happens to be the older ones, and Group B keeps taking that as Group A saying "old = better". If Obsidian got their hands at Fallout and developed one for this generation, no one would cry "nostalgia".
The problem is that Group B doesn't want Obsidian to develop instead of Bethesda, out of some contrived obligation to protect their "oh-so-defenseless" favourite developers and because they prefer pure action to complex storytelling. Group B makes up the majority and the "popular opinion", preventing Obsidian from getting their chance, so Group A can't prove that Obsidian could do much better. Then after that, Group B are the same group to cry "nostalgia" at Group A. That's why we end up with Group A sites like NMA bitter at Group B's existence, because they criticise Group A for something (nostalgia in this case) that is a result of their own actions (indirectly not allowing Obsidian to prove it isn't nostalgia).
The best solution to combat this argument is that Obsidian must be allowed to develop a Fallout that can clearly stand in the modern age without losing any faithfulness or spirit from the original Fallouts. This will give Group A a game they want, and because this Fallout is modern, Group B will be unable to use the "nostalgia" argument anymore. The situation right now is that Bethesda will not allow Obsidian to develop the Fallout that will solve this argument, and therefore Group A is also bitter at Bethesda.
My personal view is that the nostalgia argument will always be pointless. It will come again every new generation. Once we enter the next one, Bethesda will make the next Fallout for Group B, and Group A will stick with Obsidian's last generation Fallout, and Group B will begin using the nostalgia argument, ALL OVER AGAIN. That is why it is an invalid argument - the only reason Group A happens to stick with older ones is because Obsidian aren't allowed to make the first one for the generation.
Essentially, if Fallout: New Vegas was made before Fallout 3, we wouldn't be having this argument at all.
______________________________________________________________________________________________________________________
Again, this is how I see it. If you feel any part of this is wrong, feel free to share, but note that I tried to be as unbiased as possible.