Remember: The baseline for WiiU sales in UK is really, really, really low.
So if only 1,000 more WiiUs are sold in the entire UK the week of Splatoon launch, that's probably a significant % increase. That establishes a metric that can easily spun into saying that Splatoon is a 'system seller'.
And honestly, if MK8 and SSBU aren't selling the system, what else will?
I predict NX launches in 2017 - not 2016