I can't speak for others that post that they give up and have moved on, but in my case, I don't care one bit whether or when any more DLC's come out. I have moved on, and am not hoping that it forces the sudden arrival of them.
I do agree that if that is the line of thinking, that it will not affect the release schedule one bit. Bethesda/Obsidian will release them on their own time, not the time that us fans want.
The exclusive release of dead Money on XBOX managed to put off a number of fans that were otherwise looking forward to it (myself included), some of whom may have put the game down then and there because of that. Others like me just grew bored with certain aspects that we felt were lacking in the base game. Even if a new DLC came about that rectifies this, I still wouldn't be purchasing or playing it.
IMO it would've been better to capitalize on the excitement of New Vegas as soon as possible instead of taking Microsoft's money for the exclusive release of Dead Money, then proceeding to drag their feet on announcing or releasing anything more.