I would say in 20 years time your going to have a digital only market of 75% and a physical market of 25% when it comes to "existing customers". There will be plenty of developing nations in 20 years that will be reliant on physical media and the countries who show gaming promise, could very well end up loyal to whichever brand(s) still offer physical copies at that point in time. Depends on the companies and their future business model and whether or not they really want to grow in numbers and not just dollars.
Physical in 20 years time could also look very different than it does now. Just imagine all physical games from all companies are cartridge based, using advanced tech, being produced in present worldwide optical disc quantities. Something like this could push back digital only even further into the future, since the price for physical could be the same or less than it is now, a few years down the road, plus no need for a bulky, expensive, optical drive inside every console.
