Super Metroid was two decades ago. There have been quantum leaps in the design of "mature" games since then, leaps which Nintendo has, with the exception of Retro and Monolith, missed out on.

...Quantum leaps in the design of mature games? You must be joking. What exactly do the Prime games do in regards to "mature" game design that surpases or even differenciates them from what Super Metroid did 20 years ago? Name one moment in any of the Prime games that is "darker" than some of the darker moments in the 2D Metroids.

Do you seriously expect another Retro-made Metroid to be darker than that? Do you seriously think that any difference has anything remotely to do with them being a Western, "more-flexible," studio? Come on.

The vivid clinical descriptions you get from scanning the dead and dying pirates in the opening sequence of Prime is darker than anything in Super Metroid.

I doubt very much EAD would have done this.

The descriptions are no darker than the corpses of human scientists in the beginning of Super or the rotting corpse of a fallen bounty hunter being eaten by flies. There's no reason EAD wouldn't do the same if they were making a Metriod game. I mean, they haven't put anything like that in a Mario game, because it wouldn't make sense, just like how Retro didn't put any clinical descriptions of the penguins you kill in DKC:TF. Obviously.

If EAD are so willing and able, why haven't they?

I've been saying for ages now that I want the Wii U Metroid to be from EAD. And I want Retro to be working on an entirely new IP. Of course I doubt I get either wish.

That doesn't disprove my point at all, but EAD hasn't because they simply haven't been directed to. There's no mystery reason. They weren't paid to make a dark game, so they didn't make a dark game. These are people who went to school to be masters of art and design. You think they sat down drawing Mickey Mouse all day?

They've never made an M rated game. They don't have experience, and they've shown time and again how out of touch they are with today's market.

Even if they do somehow have the ability, they may as well not have it since they won't use it.

And so, Nintendo's first party lineup lacks diversity and their sales continue to drop.

Retro never made an M rated game either. And EAD just does as they're told. They aren't out of touch with anything. They are just workers trying to get paid. They do have the ability. You're critizing whether Nintendo will utilize that ability. That has nothing to do with EAD actually having that ability. If they are willing to make Retro go from Prime to Returns, there's no reason EAD can't go from Mario to Metroid.

There's no evidence that they have that ability. They certainly don't have the experience.