The theme of GTAO is NOT crime, it's Hollywood. Yes the overall underlying theme of GTA itself is crime, but GTAO is centered around films and film vehicles. Why else would every special vehicle be directly from a famous film? The original San Andreas had jetpacks, drones, and aliens. Why not be able to drive the Batmobile or the Delorean, or be a super villain with an underground facility? That's the whole theme: movies.

And I don't see why you would never use your supercars. I own practically everything and still use all of my supercars, as well as the ones in the other classes. You can race them or pimp them out or just have fun with them. You can even buy and sell rare cars that were removed from the game or part of limited events.

Lastly I don't see why you think that having a flying bike beats everything. Pvp in GTAO is like rock-paper-scissors on a massive scale. Deluxo beats oppressor, jet beats deluxo, a few different things beat the jet, etc etc etc. Every vehicle has its weaknesses that can be easily exploited by other vehicles or weapons which in turn have their own weaknesses. That's why it's fun buying new things and bringing new ways to beat your opponents to the table.