There's no way you can disprove that the online mode wasn't a draw for Mario Kart Wii, especially when you consider the millions of people who still use it to this day. Factors like the packed Wii Wheel and all that marketing certainly helped, but I'm sure the online mode padded in some sales as well.
At the very least I think SSB Brawl's lag is reason enough to demonstrate a need for improvement. Mario Kart Wii is not reason enough to conclude that things are going fine as is.
I agree that Nintendo's online games need improvement. My point was that Nintendo realizes online games can be very profitable, and I believe they will pursue improving them in the future.
I still don't know what happened with the lag for SSBB. There was no lag for the first few months (I loved playing it online at the time), some time after that is when it got the lag.
I'm sure the fact that more and more people started playing it online had something to do with that. Plus, there are now a lot more games -- and therefore a lot more people -- which use online features, including Mario Kart Wii, which was released about a month and a half after SSBBrawl. This might also have something to do with it, though I don't know how this stuff works, so I'm not sure if that would matter or not.