This is kinda like marrying your girlfriend of 15 years.
Not a bad move though, Luigi's Mansion 3 was pretty damn good, and it always helps to have more first party studios.
The hope for me is that Next Level Games will have a decent boost in productivity. I know Nintendo lists that as a boon for acquiring Next Level Games, but of course we don't know to what extent they could possibly expect NLG to increase their production capabilities. Would be nice if NLG went on a hiring spree now, I think. Their core team is apparently pretty small (with the big amount of developers on their titles probably being outsourcing some asset creation).