I disagree with the assertion that Valve is responsible for the death of physical game copies.

First off, console games have gradually moved away from offering physical copies for years. This had the (probably deliberate) effect of killing the used game market. Earlier attempts to kill that market were driven by the proliferation of digital content that could not be resold.

Second, physical copies on the PC have not been guaranteed for a very long time. I would suggest it was actually the wide-scale proliferation of indie games that did more to kill the physical copy market than any deliberate transition from AAA studios. There are thousands of indie games out there now. Without digital copies, it would be impossible for anyone to actually purchase any of them.

And the momentum to move away from physical copies for PC gaming has been there for a very long time. No one was going to Best Buy to pick up a physical copy of minesweeper. There have been other factors as well, such as the popularization of MMORPGS which provided years of content from a single physical copy.

Really though, the idea that without Valve, the PC gaming market would still be chugging along on physical game copies just isn't realistic. It goes completely against the momentum built over the last 25 years of PC gaming.