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They haven't 'betrayed' anyone. Nintendo made their fortune from appealing to people who'd never previously thought of getting into games. In fact, they invented a machine where all you had to do was put a game in and switch the damn thing ON. That in and of itself, in the context of the 80's, is CASUAL.
They then did another classic Nintendo trait of sticking with a winning formula, all the way through the SNES era. Finally, when the Playstation came out which, I should remind you, with its overabundance of sports games, movie licenses, unprecedented levels of shovelware and generally much shorter lengthed games, made things even MORE mainstream, Nintendo slipped.
They continued to slip with the Gamecube because Sony continued to adopt the same business model of going after the mainstream audience (The fact that they were ALSO catering for the 'core audience' is really irrelevant for the sake of this explanation).
The Wii is Nintendo doing what they did in the first place - Bringing games to people who didn't originally play games.
Hate them for it if you want, but you can't act like this is some unforseen stab in the back.