How many gamers need 10Gb LAN though ?
How many gamers need even gigabit LAN? The thing is it's not the pure gamers who need 10 Gb but those who do work as well as game. Architects, engineers, researchers, and many more. Those who have servers / NAS boxes and the like. The thing is, 10 Gb Ethernet gives you about the speed of a local drive, so it makes sense to keep your precious data on a separate box where it's safer. And maybe they have more than one workstation, in which case having their data centralised on a NAS / server makes even more sense.
Also you don't need pcie 4.0 for capture cards either, as you can get them in USB format,
Maybe, but it's nice to have the choice, isn't it?