Not sure what youre thinking of but older games have demos that are recorded games. Those would work well for testing nowadays too imo. Mostly performance testing but still.
I'm on Kubuntu (so KDE), been using it for over 6 months now. I am the kind of person that loves setting up the desktop and all the apps just like I want, so it has been perfect for me, although the defaults are pretty solid as well. The default Breeze theme also is really pretty, I went through all kinds of themes but eventually settled with Breeze due to how good it looks.
I feel like partially the movement happened because some themes made the apps unusable and then instead of blaming the theming, users blamed thee app developers instead.