I agree, that certainly helps Sonic sell as a game. The issue is that it won't convince many other people to buy a Wii U. If Mario and Sonic are such similar games, the people who are likely to buy a Sonic game are those who are likely to already buy/have bought the Wii U to play Mario. In other words, it doesn't reach out to a different crowd. Helps the individual game sales, but not that of the console it's on.