That's where bundles come into play
I guess Splatoon is the least likely to hit 10mm. So, if they bundle it with the system, it would significantly increase the odds of this happening. I don't think 10mm is a guarantee for Zelda either though. Odyessy will probably hit it, assuming it is a decent game, and switch sells at least 30mm lifetime. If its a great game, it could hit 10mm even if Switch only sells 20mm.