Yes, when Nintendo systems do bad sales of Nintendo software are constrained by the low userbase however when they do great, sales of Nintendo key software are enhanced (they are by default the big must have game to buy, always in the spotlight, bundles etc.).And how many times have you seen any game sell 30M copies? Besides GTAV if there's any company that proves they can do it is Nintendo.
I'm not saying every Nintendo game has the potential for 70M copies sold if they went multiplatform. I'm saying that if MKWii sold 35m on Wii alone then that game would've sold way way more that that if it were on other consoles.
Hell, Mario Kart 8 would be giving any AC Unity and CoD AW a run for their money if it was multiplatform.
I'm not saying Nintendo should do it. Im realizing how much Nintendo software success I tied to Nintendo's often inability to make a home console a mainstream success.
It's delusional to think that MKWii could have done 70M have it be on PS3/360/PC instead of Wii (or even with the Wii included).
EDIT:
GE: about 5M (NPD)