The people you want to lure to Nintendo are too insecure about their masculinity to play Kirby games. ;) 
Besides, you're talking about the people who buy yearly CoD, Assasins Creed, Fifa, etc.

They may not play Kirby, but they could be convinced to jump on board for Zelda, Smash Bros, etc.

The overuse of Mario has become a running joke, they need to fix this to improve their reputation. If they wear out his appeal, then they're screwed.

I'm pretty sure these are the kind of people that wouldn't buy Zelda just because it doesn't have realistic graphics. Mario games always sell like hotcakes. In fact, Nintendo's most milked franchises remain dozen times more popular than their normal franchises. People like milked franchises, it's that simple. How else can Pokémon still sell over 10 million copies as if it's nothing?

Nintendo would need to make powerful hardware and nothing but "mature" games to win back the hardcore crowd. And even then everything depends entirely on third party support. 

You don't have to steal back the entire audience, just enough to expand your userbase.

And no brand is immune to oversaturation.

I'm pretty sure Pokémon is, not sure about Mario. I agree they need to make more mature games (while keeping a fine balance with their cartoony games), but that will not be enough to lure a large chunk of "PS/XBOX" gamers. Only if they get third party support would that happen. 

Splatoon is probably screwed.

To get that game out there, Nintendo would need to advertise it heavy and hit with a entertaining theme or push for it that appeals to all ages.

They tried that with the new Smash Bros games and the tag line "Settle it with Smash". I doubt they are going to give Splatoon as much of a push based on how they do most of their non-major franchise titles. Tragic but they did similar failings with Pikmin 3 and Wonderful 101 in 2013.

Any expansion is better than the shrunken userbase they have now.

Yet Pikmin 3 will reach one million sold. That seems to be enough for Nintendo to keep a franchise alive (they already said there will be more Pikmin games). The Wonderful 101 didn't have nearly as much hype as Splatoon. Wasn't very accessible either with those controls.