Bro, I don't entirely disagree with this sentiment but I would argue that open source isn't always great despite how anyone feels about it. Unless you are categorizing open source with indie games. In my experience open source games are typically not great in the slightest at all. However, I agree with this sentiment if open source is being referenced to indie games. A game doesn't really have to be FOSS to be great.
The issue isn't really games like Assassin's Creed either the issue is big companies. Assassin's Creed isn't my game but I hear the newer ones are good. Assassin's Creed wouldn't need to be open source to be good.