The more digital things we have replacing physical things the better. Less trash all over the place years later when no one wants the discs/boxes anymore except for the super hardcore, niche, minority of people that like to collect.
I do prefer digital but DRM makes me wary of when it'll fully take over since there are cases like P.T. An anti-DRM movement will probably be needed in the coming decades to keep digital content in a good place.
Streaming will probably start becoming properly relevant this decade but I don't see that only harming digital purchases. Both Sony and Nintendo would prefer as high a digital ratio as possible so they make more money so once it becomes feasible will start to move away from physical.
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.
Nintendo will for sure hold on to physical the longest of the three but the increased profit from digital sales makes me expect that they'll move away from physical when that becomes feasible and eventually just keep it around for stuff like a collector's edition. I don't see the digital growth rate for Nintendo over the past few years plateauing for a while.
As we can see, the trend is clear. Physical software is dying, just like the movie/tv and music industries digital will become dominant and physical releases will become extremely niche. The digital ratio will rise high enough that even the biggest releases will not be able to hit the 10m physical sales mark. Even the biggest releases like Mario Kart that sell 40m will be under 10m physical once they reach 80% digital and beyond.
This is good data and a great post, thanks for the high quality contribution to the thread!