Fallout: New Vegas is not a sequel to Fallout 3 and Bethesda didn't make it. Bethesda has farmed out the TES to other developers in the past, though the developers were in the Zenimax family. Just like how the relatively crappy mobile games that Bethesda farmed TES out for weren't sequels, New Vegas isn't a sequel. I think we should wait for Fallout 4 until we say that Fallout wont follow that format, which it likely will.
I whole heartedly agree that we will have to wait to see; but what I can speculate for
sure (haha yes I understand how funny that is) is that there isn't yet a statement from BGS declaring that with each new installment they start from the ground up as there has been for TES. I can also speculate that FONV will be a BIG seller... and unlike the outsourced mobile games of yore, you are comparing apples to oranges. I think that Fallout is the cash cow to make BGS worthwhile to the Zenimax boardroom, and TES is the franchise that legitimizes the developers to the world. I loves me my Fallout, and Three Dog ROCKS. What I will say is that Three Dog and Vivec resonate very differently in my heart. I don't see them as the SAME thing, I see them as different things, both of which are wonderful in their own right. What I will say, to keep this post on target, is that when we look at the development cycle of BGS, I must say for sure that there are people at Bethesda right now who are working on FO4. Could they get it done sooner that TESV? Well, yeah, considering that Obsidian did it without all the history. Yeah, it isn't a direct sequel... but what makes you think that FO4 on current gen will be a "poor" game? It could be done, in New Orleans for example, or Vatican City, or Moscow, or New Delhi.
A good game doesn't have to be TES. TES has to be TES though... don't give me some piece of [censored] and call it TES. Not that I am calling Fallout a piece of [censored]... just, honestly, I expect TES to BLOW MY F#@KING MIND and I expect Fallout to Rock My World.