Comparing tablets to the U is meaningful because they both compete for a sale. If someone believes that they can get the same enjoyment out of a tablet as they can the U, then Nintendo has a problem.
As for core games, I am all for them but are the casuals that fickle? I mean Just Dance is enjoying a pretty solid and reliable trend over its iterations, and upward curve if I may add. I can't say the same about Mario and Zelda. By the way, Skyward Sword sold less because it was a Zelda on the same platform as its predecessor, there's a pattern for that (see OoT and MM).
Lastly, if Microsoft was able to successfully tap into the casual market, it must mean that there's some kind of formula for success there.
Ultimately, the solution to the challenges Nintendo are currently facing is not "Ditch the casual", but rather "Let's not stop making games that attract the casual, and games that attract the core, and let's offer better prices, because the market is getting much more competitive". Simple stuff.