The problem with Half-life 2 and so forth is that all you have is the one story mode which is completely linear and has little replay value. Unlike Half-life 1 which had a quake like death match, Half-life 2 has single player only. Now it could sell if they bundle it with something like the Orange Box. Maybe some sort of Red Box that comes with CS:GO, Dota 2, Half-life 2 and the episodes, and Portal 2.