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 personally think that, between the two, gog is far more likely to disappear than steam.

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.