And how cheap they are as well.

Eh, 500k is far from a million. I doubt the games have sold the same amount in the e-shop as in retail stores. 30% of sold Animal Crossing NL are downloaded, so I doubt there are many games on wii u that can say the same, with exception of those that are impossible to find in stores.

500 GB for around 50 bucks isn't cheap?

I think they know it but the point is that it's optional and when something is optional it's clear that quite a few gamers won't ever buy one.
Internet connection rate for the WiiU is 80% (which is good) then reduce the amount of WiiU owners who don't have an external hard drive and you'll get to a point where it's impossible to reach digital sales that are as high as retail sales.

No, because of the limited memory available and most people probably don't have a external HDD hooked up to their WiiU.

I'd say once we see some of those games (or others) hit around 750k or so retail, then they're likely million-sellers, digital included. Digital sales are probably going to work on an inverse bell-curve, embraced most by impatient early adopters and people coming very late to the game such that the eShop games will be hard to find in retail anymore.

That's why I think sales number would be more accurate in dollars. Tough that's impossible to apply.

I agree that the digital to physical ratio right now is probably near its peak, as early adopters are definitely more tech savvy, but there is no way that it's going to be 25% on a home console.  15% would probably be considered good, especially on a console without much internal memory.  Unless shown otherwise, I'm not going to believe that most Wii U owners have decided to buy an external HDD rather than simply buy thier games on disc.

What is for you relatively close to one million? The 5 following games sold between 0.25 and 0.5 million.

Good point.