By now most of the world has probably heard about the partnership between Nintendo and DeNA. While it's reasonable to be nervous about this prospect (free-to-play is still something of a slippery slope, and Nintendo's never really done it before), there's really nothing to worry about. In fact, there might even be room for some cautious optimism.
First of all, while Nintendo made it abundantly clear that they aren't going to be porting any of their games to mobile, there's a decent chance that resolve could waver in the future. Now I'm not saying it's a definite, but Nintendo did say that they'd never be bringing their IPs to mobile. And look at them now. So while they might be insisting that there won't be any ports, that tune could change.
Secondly, Nintendo hasn't been doing so great, financially [Editor's Note: Although they're a bit more optimistic about this year]. Assuming this partnership works out, the developer could stand to make quite a bit of money (everybody loves Mario, right?). If nothing else, the potential income from their new mobile library will let them keep making console/handheld games.
Nintendo has also stated that every single one of their properties and characters could potentially see their own mobile games. That's a pretty big back catalogue to pull from. Ignoring the obvious ones like Mario and Donkey Kong, because you just know they're a given, that still leaves all sorts of nostalgic goodies like Balloon Fight, Hogan's Alley, Excite Bike, Clu Clu Land, Gyromite, F-Zero, and so on.
But the biggest reason you shouldn't let any of this worry you is that Nintendo's current and previous games aren't going anywhere. No matter what comes out of this partnership - good or bad - none of it will invalidate the Nintendo games you already know and love.