brunosiffredi: I think you are missing the point. Why does Apple remove apps from people's accounts when they get removed from the store? Because there's a cost of maintain user access to old software that isn't bringing in revenue anymore.
It's a matter of maximizing profitability. If Valve ever goes public with their stocks, you bet they'll start finding excuses more and more to remove old games from people's accounts.
Pheace: Considering GOG is the one that's actually publicly traded they're the ones at a higher risk of doing so. Personally I'm pretty sure both of them will avoid it like the plague since doing it will mean they'll start losing future sales.
The difference is GOG doesn't have a reason to do it. The user base is much smaller and there are 1340 games on the store right now. Even if we try to add to that games that were on the store and got removed, the number wouldn't reach 2.000 games.
Steam has something like 15.000 games on the store right now, with more or less 20 new games being added on a daily basis. It doesn't take a mathematician to see that it won't be sustainable for them to keep every game they ever sold on their servers forever.