Actually, Nintendo's going back uphill. Their systems, except for the Wii, have all been more powerful or rivaling in power compared to their competition, technically speaking. Nintendo just decided to go super casual with the Wii. The Wii was something bittersweet, to tell the truth. There were a handful, maybe even two handfuls of awesome games for it, such as Monster Hunter Tri, No More Heroes 2, Super Smash Bros. Brawl (obvious), Super Mario Galaxy 2 (nine year olds can't beat this game 100%, so it ain't JUST for youngins), Tatsunoko vs. Capcom (MVC3's control scheme was based off of this), and not to mention the lineup of Virtual Console games. To sum it all up: the Wii wasn't really Nintendo's downhill moment, it was their "valley" moment.
P.S. - sorry for that über long post. I just tend to get carried away.