It should only be on the onus of the video card manufacturer to provide a support piece if needed. Motherboard and case manufacturers shouldn't be the ones to do it, since it it could potentially limit designs for something that probably isn't in the top 0.1% of all video card ownership.
Besides, motherboard manufacturers are doing their due diligence by adding structual support to at least the primary PCIe x16 slot.
EDIT: And there's a whole market for these anyway:
https://www.amazon.com/gpu-support-bracket/s?k=gpu+support+bracket