Does anyone sell a conversion kit to install rim brakes on disc frames? Probably not possible with cantilevers, but might be for road calipers.
Also, complaining that GRX isn't available with rim brakes is a bit like complaining that new computers don't have CD drives. There are plenty of workarounds and other options if you want to stick with rim brakes, but I wouldn't expect manufacturers to continue catering new products around them. I have SRAM Force 1x on an old canti CX bike right now. It works great (the drivetrain... not the brakes).