I get what you're saying, but I don't think Pikmin would really sell to "casuals" anyway. People who care about both will buy both, people who only care about Mario won't buy Pikmin if its their only option.
Agreed. A scenario I posted in one of the old Wii U spec threads:
Hypothetical example of why Mario is needed at launch:
Soccer mom goes into Best Buy during launch.
"Oh what's this Wii U system, that looks kind of interesting. Does it play Wii games?"
Sales guy: "Yes, it's compatible with the old Wii games and controllers, plus it has a new tablet style Gamepad."
Soccer Mom: "Oh, and that looks cute, what game is this."
Sales guy: "That's Pikmin 3. It's a strategy game with little plant people."
Soccer Mom: "Can the family play it together?"
Sales guy: "Well, I think it has an optional 2 player mode."
Soccer Mom: "Hmmmm, ok. Oh well, I think I'll get my son an IPad instead and he can just keep his regular Wii which he still plays a lot, we all still play that Wii Party game too. But what's this new Super Mario game, will that play on the old Wii?"
Sales guy: "No, that will only play on the new Wii U system."
Soccer mom: "Oh really? My son is a huge Mario fan and won't want to miss that, and his sisters like Mario too. How much is the Wii U system again?"
System sale gained. Other scenario with Pikmin 3, third party ports but no Mario game quite possibly another result.