I think it's perfectly realistic to think there is a substantial risk of losing library content you've bought on Valve in the next 20 years. Don't know what the odds are, but they're greater than zero.
I'm happy both exist. I've nothing against gog (except maybe for their broken promises around Linux support, but I do understand changing market forces) and like I said, I hope they succeed. They've got a good mission.
If Steam disappears, your games will become inaccessible.