Mr. B Natural said:
I didn't say sales, I said return on investment.
Your argument completely ignores the possibility of limited opportunity windows, though. Sure, if Nintendo literally had the decision of getting back an $X dollar return for each dollar spent on SMG games or $X x 3 dollars back for each dollar spent on Wii * products, yes, it would be a bad investment, but Nintendo can't spend unlimited dollars on Wii * products -- they need a new, unique, and
good hook for each one, they need to avoid saturating the market or turning off their new expanded audience with a bad release, they need to have a clear space on the calendar in which to extensively market it, etc.
Given those factors, I think it's quite likely that there's basically a cap on how much money Nintendo can actually get a nongame-sized return on during any given year, and their goal then has to be to find a diverse set of options, each with a decent return themselves, to spread the remainder of their investment over. SMG2 -- an extremely un-risky release with a reused engine that is nonetheless going on to sell multiple millions worldwide -- strikes me as a very good choice as part of such a strategy.
(This is also why the idea that Nintendo would have ever given up on "real" games entirely and made only nongames going forward never made any sense to me.)