I voted never.
Nintendo is at odds with other software publishers, because they pursue a philosophy of maintaining high value for their software instead of going for games with built-in expiration dates to make people buy something new when the old is no good anymore. High value goes hand in hand with physical copies.
You also have to consider that a lot of formerly digital-only games received physical versions later on because all-digital is clearly not the future. Another factor is resell value and an ever-growing retro scene; every console will eventually become a retro system, so physical is also an investment in the future.
As this prediction is defined as only one single game needing to accomplish the 10m feat in any given decade, it's hard to imagine that we'll ever get to the point where no game manages to sell 10m+ physical copies.