Gimme one,
On-Topic:
I wouldn't have bought OB if it weren't for modding,
and I don't think I'm the only one
Personally I don't think that initial sales would decrease much if the next TES title wasn't moddable. Probably nothing worth mentioning. We didn't even know whether FO3 would be moddable, but (as far as I can tell) it still sold extremely well before they announced the GECK. The majority of people don't even use mods (consoles) and even the ones who do would probably buy the game anyway. I know I would, just to see what it looks like.
But I think the longevity is what matters. Not because people come back to play the game years later. They already bought the game, so Bethesda isn't making any money with them (except for a few who switch from console to PC maybe). But the advertising effect is huge. I remember that in a (rather) recent interview Pete Hines said that Oblivion and Morrowind still sell great 'because our sales guy stays on top of it'. What a joke. Just google
any word and you'll end up with a link to an Oblivion mod. No sales or PR guy could do what the billions of links to images and sites do that only show up because Oblivion has so many mods. No matter what you want, be it Star Wars, Halo, Anime, Lord of the Rings - you end up with Oblivion. You can't avoid the Elder Scrolls titles when browsing the internet. That is nothing you can do with regular advertising efforts, no matter how many million $ you spend.