I think that after so many years, a game should become free. Just like movies or comics once a videogame is 5 let alone 10 years old, younger generation gamers won't want to pay to play such old games. Even now I doubt I'd go and buy the original Starcraft when there are newer and better games available. I think that after 5-8 years a game should become free. The only people who even make money off old games are places like videostores who have them all heaped into a bargain bin like trash for 5$ a game. Reasonably by making the games free to download (While still just selling physical copies for like $5) it'd allow more people to experience and enjoy the games that we've grown up with. 10 Years from now I doubt I'd even consider tossing in Assassins Creed, Fable, or even Half Life 2, by then I'll be too pre-occupied with the new games and I wouldn't want to bother paying for those old relics. But if they are free downloads and cheap copies, then it's likely that more people would actually go back and play them. Heck, I wouldn't have ever played FF7 again (Last time was when I was like 8-9) unless I had bought it off PSN (Pretty rare for me to even buy games online).
Basically, free/cheap = preserving the games of the past so that future gamers can enjoy them. While, paying money = less likely gamers of the future will want to bother, and less likely that older gamers would want to spend the money on stuff that is considered outdated.
Edit: It should also be noted that the game companies themselves get ZERO dollars from games that are several years old. The companies make money from the purchases by stores and gamers within the first year or two, but after that it basically goes straight to the store. That's why when you trade in a game for $5 they can re-sell at $30, because you've already paid $60 upon release and some of that went to the developers, now you've recieved $5 out of the $40-50 the store is now getting. Hence the reason why some stores will still sell games for $40 when the company is offering the game at $20 on their website for Download or ect.