Hardware

"new" "major" feature to help sell the new cards. Let's say you got a 4090, upgrade your panel, and then realized you're gonna need DP 2.1 because you got a 20k super HD oled monitor... time to drop $ for that 5090.

There is "Display Stream Compression" or DSC that I believe can help with the bandwith constraints from 1.4a to 2.1 but it's compression and may cause latency/quality loss - in general you just lose future proof as AMD already has DP 2.1 meaning they can fully support the 540hz monitors for example or any high quality panel.

The refresh cards still use the same silicon, just with a different amount of disabled functional units. Enabling DP 2.1 might require an expensive partial redesign, which Nvidia doesn't really have any incentive to do, since people will buy the GPUs anyway.