I wouldn't call it dying, but it's definitely niche.
From a business standpoint, it's more profitable to sell prebuilt computers than it is to sell the individual parts and have the customer build it themselves. Radioshack probably ended the service because it wasn't profitable or important enough to keep while the company was in its death throws.
As far as I know, there are still services that will build custom computers for you, then mail them to your house.
If computer construction was entirely modular and simple as plugging in a few wires, it would be great for the industry and would create a much more personal connection between the user and his or her computer.