There were "mainstream and popular games" for System 6 as well as System 7. You will be surprised at what still runs under System 6. For example, Deneba Canvas 3.5 runs wonderfully under System 6.0.8! System 7 may open compatibility to certain "popular" games that won't run under System 6. But you get a lot more speed out of System 6 than you do with 7.

But there's no need to just worry over it. Just put System 6 and 7 on your machine and switch between them as needed! That's what I do via System Picker. It's easy and convenient.