I have a 3Mbps DSL connection. You can BEAT a disc-based game in the time it would take to finish a 20GB install. 20 GB = 21,474,836,480 bytes. That is 171,798,691,840 bits. I get 3 Mbps, or 3,145,728 bits per-second. That install would therefore take 54,613.3 seconds. That's 910.2 minutes. That's 15.17 hours. I can literally beat the entirety of a game fast than that. In some cases, I could beat TWO games faster than that.
I think you need to call your cable company and check the rates, you might be surprised. Maybe you haven't looked into the pricing lately.
On another note, 20GB = 15 Hours. So is it easier to drive to a store and buy it and not be able to have all the cool DRM features. Or how about this, you preorder it digitally. At midnight your XBOX ONE would do an automatic download and by the time you were home from work the next day, you could play your game. Or if you don't work it would be ready early afternoon, by the time you woke up.