Linux has more running in the background now than it has before and a lot of us turn all of the extra crap off when we need all the resources...but yes your point is valid and that is why it is frustrating to still need Windows to game.
I'm not sure what you mean by "most replace that with open GL". You can't replace what the game was written to use and most of the high end games use DX10 or 11.
Anyway, I used to spend hours tweaking my config.sys and autoexec.bats even playing with jumpers to get a game to work, and I enjoyed it. I still have special boot disks I made just to play Privateer. These days I just want to start a game without any fuss and use my increasingly small amount of leisure time actually playing. On a similar note I don't want to play only old games. Will I be able to buy splinter cell blacklist for Linux? No. Can I play some game made 5 years ago on Linux? Probably, almost always if I'm willing to play around with it.
That isn't good enough.
You and I approached this same problem in different ways. Since my leisure time 8is* getting increasingly rare, I don't want to spend the money on big-time expensive games. I'll play something simple and entertaining while I kill a little time.
I do play Civilization V, but nothing more complicated than that, technologically speaking.