I think some peoples expectations for splatoon are WAY too high. Just being a shooter doesn't give you mainstream appeal or high sales and splatoon barely qualifies as a shooter. Honestly, the content in this game looks lacking and the multiplayer is very limited, especially for a shooter.
Personally, my expectations for splatoon right now is that it is going to get savaged by reviewers, probably 6-7 meta at best, and have sales in line with W101. If it does better I will be pleasantly surprised, but everything about this game that I have seen just looks like a indie game that they are trying to push into a full retail package...