That's where I think you are a bit wrong. Like an earlier poster said casual gamers who bought the wii and would have bought the wii u are now buying tablets to play game. The whole point of the wii was to target those gamers who consider themselves casual and like the familiar faces in games. The tablet market literally stole the potential wii u buyer. This is not the same as Xbox and ps3 gamers, yes some may have went the tablet way, but not as many as the Nintendo gamers. You really can't get the same experience playing on an ipad as you can playing on a PS3 or Xbox. Most xbox and PS3 gamers get it for because of the exclusive titles and the online experience. Titles like Call of Duty will already make the demand high for the new generation consoles, compared to the wii u mario cart. Yes the wii did have COD, but the sales for COD on the wii were no where close compared to xbox, pc, and ps3. I personally don't like the game, but even i can see how far people are willing to go to play there favorite games.
I have a couple of friends who don't touch their wii's ever since they purchased there tablet. Those same people I would consider casual gamers who liked the Nintendo series and never owned a PS3 or Xbox.
tablets are not taking potential wii u buyers away. the crappy economy is doing the job. the economy was much better during the original wii's release, and the original wii was also priced lower. people who want video games dont choose a tablet over a wii u... for the same reason you said, that you cant get the same experience.
im sure we will also see lower sales numbers for the next xbox and playstation than the previous generation due to the economy... unless those systems have something incredibly innovative like the original wii.