Thinking that an M-Rating is a sign of Nintendo growing up is a sign of immaturity. Additionally, Nintendo has no obligation of "growing up" when both their fans and their detractors throw tantrums over every little thing. Making it have excessive blood, gore and nudity is pointless to the gameplay and exploration.
One of the problems I see from this lust of M-Rated games, is that many of these games over value an edgy story over gameplay and exploration based stories found in Metroid. I remember I made a XCX about voice acting, and someone mentioned a rated M Metroid, which I was mulling over until I came to the right conclusion and that person mentioned sex scenes.
Also I'm not sure if you ever played a Prime game, much less any Metroid, but they don't drain bodily fluids they drain life energy. This is specifically stated. It's why they can drain energy from from the Varia Suit as well as mostly mechanical. Due to that, they can also transfer that energy. It approaches concepts of energy that is the very basis of the plot of Metroid games with Metroids in them (which is basically all of them). Something that couldn't happen if they were merely M-Rated blood suckers.
