Doubt this will be available at launch day, but I could see it coming not soon after. Nintendo is already letting XBox Live onto their platform. Why not stadia? Only reason might be if they have a secret deal with Microsoft to combat Google. Very likely that Nintendo will allow Stadia too though.
Then, yeah, Switch becomes the ultimate gaming platform, all Nintendo first party plus all third party games on one device. It gets bonus points if you can also use any Switch controller with Stadia instead of the Google one.