I like GOG and I like steam too. While it is true that GOG can’t take the offline installer from me, this does not make it true I can play the game forever since many games are dynamically linked to libraries that may not be available in the future. This happened to me with games I just had bought. Steam also dynamically links to libraries but what I like about the way they are doing it is that these are part of the base installation so as long as you keep these files, the games should keep working. Nothing being perfect, I think they both try to do things in their own way and try to convince us that it is the best one.