There are already multiple 21:9 monitors that are 38-40 inches in size. LG even have a 45" model. So it's not unlikely to happen, it's already happened.
The only other requirement is the resolution, and we already saw from the leaked panel roadmaps that 21:9 ultrawides at 5K2K resolutions will be coming in volume in the late 2024-2025 time frame. Whether they will be mini-LED or OLED is yet to be known.
Dell have a display available now that checks most of the boxes in the UltraSharp U4025QW: 40-inches, 5K2K resolution, 120Hz, 5ms response. It's almost perfect, except for the $2,400 price tag.