Says who? Who exactly gets to say that Retro is the only studio capable of delivering a system seller game or "better than anyone else." You're logic is embarassingly flawed. Retro's job isn't to make you Metroid games. Their job is to make good Nintendo games like every Nintendo studio. Nothing is a "waste of their talent." They don't get special treatment.

DKR2 is just as good a use for their talents as anything else.

Nintendo has a dozen studios that could make DKR2. But Retro is their only studio (besides Monolith, who are busy with X) who has shown a proficiency at creating the kind of heavier, darker games that could appeal to the PS/Xbox audience and bring in new customers.

Retro has worked on two Donkey Kong games and Mario Kart. It would be stupid not to choose them to make a Donkey Kong karting game when they are clearly one of the most qualified to do so. Nintendo has a dozen studios that can make Metroid. Just because they hadn't made a "darker" game before, doesn't mean they couldn't now. Just because a team has only shown talent for one thing, doesn't mean they don't have the talent to make something different. Ie, Retro went from Prime to Returns. Nintendo makes light games by choice, not by lack of available talent. If the NSMB team were asked to make a 3rd person shooter M-Rated Metroid game, they wouldn't release Mario with a Metroid skin a year later. They'd do what they're told, because they'd have a job and the talent to fulfill that job.

I don't care about Diddy Kong Racing, but I want to see a Donkey Kong Returns 2 and 3.

Also, if this rumor come true, I hope Monster don't screw with the controls of 3ds version, like they did with DKCR3D.

Retro's the only available team who've proven they are capable and have experience though.
They're also Nintendo's only Western first party team, which allows them to bring a more Western flavour that the Wii U's exclusive library is sorely lacking.

Who cares what they've proven? They hadn't proven anything when they first were given Metroid but four abandoned game prototypes. Still made an amazing game. Any other Nintendo studio would be in a better position than they were are the time. They may even make a better Metroid game.

Metroid is a japanese franchise. It doesn't need western developers to be good or appealing. And even if it did, Nintendo could just hire a western studio to make the game and have someone from Nintendo oversee the game's developement. It's basically what they did with Prime anyway, only that studio had nothing to show for but 4 failed and abandoned game prototypes.

Nintendo can no longer survive on colourful cartoon games alone.

They need to bring in new gamers from the PS/Xbox crowd to survive. Retro is ideally placed to provide these, and as a Western studio they can appeal more to the Western audience, which in today's market represents the lion's share of gamers.

You don't get your A-team to make a game that won't move hardware. Not if you've got any sense.

Retro isn't their A-Team. Retro is just a team. Retro has never "brought in western gamers." Metroid Prime has always been a Nintendo game for Nintendo fans alone. It never appealed to any other market, and wishing it now won't change that. Nintendo can still make dark games and have Retro make DKR2. Metroid servived before Retro and it will servive without them.

Show me the last time a first party Nintendo game besides Retro and Monolith made a "darK" game.

You're seriously fooling yourselves if you believe that a new Metroid game would move a significant amount of systems. We're looking at a few hundred thousand units here (LT), perhaps 500k if the planets are perfectly aligned. Diddy Kong Racing may not be the game that Wii U needs, but Metroid most certainly isn't what Wii U needs. If you want Retro to work on something that could actually help Wii U in a meaningful way you should hope for a Retro developed Wii Sports title, a CoD clone, Pokémon mmo or pretty much anything but Metroid.
Just admit that you want a Retro Metroid because of the very reason that the Metroid trilogy was beyond marvelous, and for that reason only.