RPG veteran Josh Sawyer says Black Isle Studios' stab at Fallout 3 was never going to be made in time: "With the staff we had it would have been at least two years"

gamesradar.com:

Fallout: New Vegas director Josh Sawyer says that Black Isle Studios was never going to complete its second canceled Fallout 3 project in the timeframe that'd been given for it, saying it would have taken "at least" another two years with the team that was available.

Although Fallout 3 is, of course, a game that exists, two attempts at a third Fallout game were made by Black Isle Studios before that final version by Bethesda ever came to be. As revealed in an interview with IGN from 2017, the first actually ended up leading to the creation of the RPG Icewind Dale (on which Sawyer was a designer), but the second, and more widely known attempt was codenamed Van Buren, which was canceled in 2003. 

This decision to cancel Van Buren was apparently fully finalized after Fallout co-creator Tim Cain was brought in to test out the in-progress RPG, and was asked how long he thought it'd take to complete and ship it. Cain suggested 18 months, noting that he didn't think it'd be possible to do it in any faster than 12 months even with a "death march crunch," but was then apparently told by the vice president of Interplay that any answer over six months would have resulted in the game's cancellation. 

In a new thread on Twitter, where Sawyer discusses his "useful skill" of being able to tell when a game won't be completed "with close to 100% accuracy," one Twitter user brings up this story of Cain's consultation on Van Buren. Sawyer responds, confirming that the RPG was never going to be completed in six months, and suggesting that Cain's 18-month estimate may not have been enough, either.

"Honestly his 18 month estimate was optimistic," Sawyer says. "With the staff we had it would have been at least two years [in my opinion.]

"I don't think Tom French [Fallout 2 programmer] or I had ever suggested nine months was feasible."

It took another five years before a game called Fallout 3 was ever released, even if it was a different one made by different developers. Even if Van Buren