How quickly physical games disappear from the market is entirely up to console companies and video game publishers. Publishers almost unanimously agree that strictly digital sales is their preference. With digital endeavors, there won't be any need for the middle man. (retailers) Alot of money also gets saved by cutting out shipping costs and the retails industries cut of the sales profits. It's not a question of "IF" it's a question of "WHEN".
The question we SHOULD be asking is if internet in the United States will be ready for a video game digital market. Because as it stands right now; that answer is "Hell No". Internet companies, their shitty business practices, shitty speeds, and the overall average speed of internet in American households is no where NEAR ready for the digital age.
And if they (video game companies, consoles) rush into the digital market too fast - the people who will suffer from it will be almost entirely gamers.