This is Nintendo we are talking about, so sales do not really determine how interested they are. Remember, it took them six years to release a sequel to NSMB and if the 3DS wasn't a failure, then NSMB2 would have come even later or perhaps not at all. It's basically just a question of "do they want to make a game or do they not", because games with lower sales potential than Metroid aren't uncommon.
The worst thing about this is that giving WayForward a chance on Metroid (as well as giving other third parties an opportunity to work with Nintendo IPs) would benefit everyone, but Nintendo doesn't seem to see it. It would strengthen Nintendo's bonds with a given third party developer, it would increase the number of interesting games on their system (thus keeping their business more stable) and gamers would see new entries to their beloved series more often.
One problem that can arise with outsourcing is that the developer might be crappy, but here we are looking at cases where great talent is actually excited to work on these games. WayForward would like to make a Metroid game, Platinum Games would like to make a Star Fox game. Nintendo themselves is currently not interested in making new entries to these series, so it's win-win-win all around.
The best thing that Nintendo can do with their money is to invest it into game development, because that way they are going to make even more money. But during the last two or so years it seems that they've looked at anything but games to expand their business. It's time that they get back to what made them great in the first place instead of pursuing non-gaming opportunities. I hear stuff about Miiverse and Nintendo TVii and have to ask myself why is Nintendo making it so hard for me to buy a Wii U. Just where are the damn games? ... I better stop now, because I'll just get too angry otherwise. It's going too far off-topic as well.
I support the idea of giving game developers a chance to develop Nintendo IPs... I love that with Wii U we are seeing healthy collaboration (Ninja Gaiden 3 is a much better game this time, Tekken has Nintendo-themed suits, Bayonetta 2, even Super Smash Brothers.)
Specifically, with this situation, a game developer like WayForward has not proved themselves because of several decent games in the eShop. A small developer that Nintendo should continue to support through eshop promotion for sure, but I'm not ready to pay retail price for any of their games yet.