I disagree. Your rationale is that a causual gamer will not spend money on subscription gaming. They will if the price is right and the games are good. People will pay for a good product, period.
You make a good point, though.
My only real point is that Nintendo is over-estimating how many users they think will sign-on for paying to play online.
Another thing to take into account, is that there will not be a set subscription or price. The amount of money needed to play online under this system will vary with how much you want to play online. It would actually be better if there was a set subscription like an MMORPG. Instead, you will have to limit how long you play online due to how many Wii points you have left or can afford to purchase that month.