I think there really was a time when gaming seemed like a fundamentally limited medium that was really mostly for children. Non-bankswitched 2600 games, for example, are not really a medium that can be used for very much interesting storytelling, just not-very-repeatable (unless you're 8 years old) "flashing colors" experiences. I agree that it wasn't long after the 2600 era that this changed dramatically though. In the US, it realistically was the success of the Commodore 64 that changed this. 64k of memory and much more capable video hardware completely changed the game. Similar machines led this revolution in Europe.
Actually, even before the 2600 there were complex mainframe and Unix games (for the very limited audiences that had access to these machines), but my point is that I can't really blame somebody in, say, 1978 seeing an Atari cartridge game and thinking it would never be something adults were interested in.
BTW, do you know the 1983 book "Pilgrim in the Microworld"[0]? It's a truly insightful and deep critique of Breakout and playing video-games in general, from the era (and machine!) you're speaking of. I can only recommend it!
Actually, even before the 2600 there were complex mainframe and Unix games (for the very limited audiences that had access to these machines), but my point is that I can't really blame somebody in, say, 1978 seeing an Atari cartridge game and thinking it would never be something adults were interested in.