My current computer is about 3 years old and the graphics card I put into it 2 years ago is starting to go out. I'd much rather have a new machine than just a new graphics card. Also, I'm not able to look and see how many watts my power supply is as its hidden and I don't trust myself to remove it and get it put back in correctly.
Removing a PSU is extremely simple. You just unplug all the cables and undo 4 screws. People often don't realise just how simple building a PC actually is. If you can read & research a bit with Google to make sure things are compatible, and use a screwdriver, you're good to go. It really is very, very easy.