I would say that both variety and popularity are important. Again, I understand the point that Donkey Kong Country Returns was Retro's most successful title. I'm not sure what the most recent LTD figure is for it, but I know it's sold at least 4 million, so I'm just going to go ahead and say it's probably around ~5 million, if not a little more. I'll happily correct this if someone has a better figure, but for now, I'll go with 5 million. That's a strong sales number.
So, why does anyone think going a different route might -- and I stress MIGHT -- have been a better play (whether we're talking about Metroid or something else)? I think the reason is because the system already launched with New Super Mario Bros. U, the sequel to a game that sold nearly 28 million copies. For fans of 2D platformers, another Donkey Kong Country game from Retro is great, and I'm legitimately happy for those of you out there that are thrilled that it's getting a sequel. But for those out there that thought that Retro's mystery project was going to be something different -- that it might have had a chance at really affecting the sales needle -- it feels like a missed opportunity.
You don't have to agree. You don't even have to care whether or not it's a legitimate system seller. It's perfectly fine to be thrilled that it's a game that will most likely be great and will perform well. However, my position in regards to reading sentiments that Donkey Kong is the obvious choice with no downside is to squint uncomfortably a bit and reluctantly argue that -- at this point in the Wii U's lifecycle -- they really could have used something different. Whether the current team at Retro could have handled a big title along the lines of a Metroid-type game, I think the Wii U could have used a title like that that Retro has historically proved capable of producing.
Alas, the situation is what it is. Donkey Kong will surely be a great game, and for those that care about sales figures, we'll just have to wait a while and see how the November figures shape up.