I'd say there's more weight in the predictions that the current or next generation will be the last ever generation, than in any argument that we'll see the release of the next Sony/Microsoft consoles as soon as 2018.
And I don't have much faith in that particular prediction.
If Nintendo were to bring a new console out in 2017 (unlikely) it won't incite MS or Sony to release early. Nintendo's new console would probably be about as powerful as a ps4, but with no games other than Mario Kart 12 or whatever number they're up to - and that'd only come out two years after the console. But of course they'll have the facilities in place for you to re-purchase your favourite classic games in the meantime, so you can pay yet again for the games you've already bought.
No, I don't think we'll see a new console release from MS/Sony before 2020. And if Nintendo are coming up with something "new" some time soon it'll be a product that they've already released but in a new colour or with a new ram-pack screwed onto it or some such typical Nintendo bollocks.