Except the fact that the game is still being played brings in a small amount of sales. Myself, and others have lost numerous CDs and CD keys over the years, especially during moves. We just buy new copies (Blizzard's new online system lets you store your CD keys and digitally download, so this will change some). I've bought 2 physical copies and one digital for both Starcraft and Warcraft III. Multi-player is the sole reason.
The benefit of having the game good enough that people play it 10 years later is that people have friends. Friends convince friends to try out this old, but really great game. That means the game still makes money while lesser games are no longer even sold.