New hard drive. New RAM. New battery. Probably want a new CPU, but that's not upgradable. Ditto display. Can't add Thunderbolt, USB3, or whatever nifty new interface.
May as well just get a new computer at the 2+ year point, seeing as you replaced half of it and want to replace the other half.
"This here's a good axe. It's had nine handles and three heads."