When you buy a game on a CD or Cartidge, it’s up to you to make sure you continue to own it from then on. That is the same model as GoGs digital downloads. You own it, you make sure you still have it on hand for as long as you want to still have it on hand for.
Whatever words you want to use, when steam decides you don’t own a game anymore, they can take it away. I’ve had 2 steam games taken away from me.