When a sequel loses the core fanbase of the series, something is very wrong with it.
Lol, you say that like the only "core" fanbase is NMA dudes, when there's plenty of people out there that played F1/2 and liked it, and then played F3 & NV and liked it even more
They are not the core audience. I'll try again
Imagine if Relic made
Dawn of War III, and the game was for all intents a ~
Space Marine clone. That game could be awesome, and even loved by Warhammer 40k fans ~but...
That is not a 'Dawn of War' title, and not played for reasons a fan of DoW would seek out a DoW sequel; as opposed to playing just any other Warhammer title. I think FO3 is a good game on its own, for what it is ~mechanically, and artistically, but there is nothing left of the Fallout franchise in FO3/4 that I would want or look for in a Fallout sequel... and I would actively seek out (with an open mind) any game purporting to be one.
Bethesda getting the license was the first I'd ever heard of the company; so I went out and bought Oblivion to see what they were capable of doing with their own in-house IP, and I was immediately impressed with their engine ~specifically just how amazingly well suited it was for building a Fallout game from; they wouldn't have to change it much, or start from scratch. It was absolutely beyond conception ~for me, but not others here, that they would actually/seriously/unbelievably... forgo making Fallout 3, and instead making Oblivion 2.5, but they did, and skinned it with the least significant trappings of the Fallout IP.
When I saw the first PR images, I thought they had matched Bladerunner in their interpretation of the source material; when I saw the first gameplay screens, I was overjoyed... and mistaken (it wasn't them).
When I saw Bethesda's [authentic] first gameplay, it felt like a kick to the teeth, and a parting slap. It felt like I'd been one of those fans waiting days [years] at the door for a midnight release, and minutes before opening, they bused in a gaggle of clueless preferred customers, and ushered them in ahead of the line. They abused the Fallout fanbase for word of mouth/hype, and pulled the rug out at the last minute like some insane Jamie Kennedy bait & switch prank.