> Is it all to support the biggest and baddest high end GPUs that cost more than the rest of the system?

I think it's more to have a big window with lots of RGB LEDs to show off on the internet.

Newer SFF cases from Ncase/Formd/Louqe are designed with perforations or mesh on every exterior surface to maximize air flow. They can support an air-cooled 5090 and an AIO or massive tower cooler for the CPU. Put a 1000W SFX PSU in there and I don't know if you'd really be wanting for anything spec-wise.