you underestimate how much money bethesda games make
I'm fairly sure that if bethesda somehow did forget up fallout/TES then they'd definitely be making significantly less money on their games. Skyrim still tops the steam top sellers chart every once in a while, that's because it was a really good game. So good that 4 years and a few months later it still sells.
If fallout 4 doesn't do good that means bad things for bethesda, even if they still make loads of money. The next fallout game, elder scrolls game, and whatever else they release would be put under heavier scrutiny over the failure of such a hyped up release. If fallout 4 fails that means bad things for bethesda for a long time after, assuming that they're aiming to make another "skyrim".