Define "full use".It's not even about that per se.
is about buying a 4090 just for gaming and thinking games are designed to make full use of the hardware not only for performance but something more meaningful like mind-blowing mechanics and/or level design.
4090 can definitely be fully leveraged by games. Level design isn't something a gpu would help with anyway, unless the game is using some realtime procedural generation a la dwarf fortress and it was specifically coded to be done via shader math. The problem is that games sell off of having fancy graphics, they don't sell off of having crazy good AI or new game mechanics. Look how long it took for the souls formula to take off.