Borderlands 2 shipped 8.5 million. Even if we assume 2K overshipped, we're still talking sales of somewhere over 7.5 million copies. It's no CoD or even BF, but it's no small demographic.
And no offense to Borderlands, but I highly doubt we can take Borderlands as the exemplary title to represent the maximum potential scope of an audience looking for games like this.
I think Destiny is easily going to grab the Borderlands crowd, and more than that. It's clearly being pivoted as something that's fundamentally delivering the Borderlands experience, but also have slices of something for everyone else looking for a different type of experience. Thus Bungie occasionally refusing to call it an MMO, but shared world that can have a end-game single player experience, co-op, competitive, exploration, etcetcetc. Destiny is being sold on the notion that it's delivering a lot of extra things, on top of the Borderlands loot-shooter.
Whether or not Destiny delivers on those 'extra stuff' is, of course the $60 question. The reality is that there's very little new footage of Destiny outside of the basic gameplay we've seen from the E3 demo. We haven't seen the town hub, how it functions, how vehicles work, how exploration scales... basically the thing that will make it sell better than Borderlands are still promises that have no videos to back them up yet.
I expect Destiny's blow-out to be at Sony's E3 pressy this year, where Bungie needs to finally answer the question of 'what makes Destiny more than just what they've shown?'
