Absolutely, GOG is my god and savior.
There is an issue in that the games industry moves at such a pace that catching up makes older games harder to sit down and play (because of time constraints), but with rare exceptions, I don't ever play the newest releases; so I guess I only play old games?
Right now I'm playing FFXIII. It's an old game, but I only got it now on Steam (the port is recent, too). Before that I was playing Inquisition, which is two years old. I have yet to buy GTAV, so when I do play it, does it count as old? By old, do you mean purely based on release date or generation?
I also play Neverwinter Nights, at the moment. I was playing Fallout and Baldur's Gate before that. Or Jade Empire.
And then, how do MMOs factor in this, since a good deal of them are old? WoW is gigantic and favored, but it's definitely old. At the same time, it gets expansions frequently. So, does it count as old? These days I'm trying to get back to Dungeons & Dragons Online. Definitely pretty old, but it's still going strong.
Most of the PC market, particularly the core gaming side of it, probably play older games for a number of reasons. You're right though; just like with movies, a good product is good regardless of when it was made. It's actually kind of rare for a game to be entirely obsolete, because a newer game does everything better and offers a better experience.