Its like this with many antiques these days. Restoring them can actually decrease their value. Its usually on things that have very few original remaining examples left. In the case of guitars there are sometimes only one example of such and such guitar played by whoever at a certain concert. The same goes for antique cars and motorcycles. I collect Indian motorcycles and its pretty rare to see an original paint bike but they are out there. Even when they are completely rusted and exempt of any remaining paint its still worth more in an unrestored state.
There will be a time where an unrestored example of many pins will be rare. Im sure there already are plenty of rare titles that fall into this category. If you have a game that is in great original condition I would leave it as it is. My xenon is about a 90/100 as far as the cabinet and backglass goes. The playfield is shot and is getting a new PF but the rest I will leave. Its all preference really. Some love old patina and some love new restored and shiny. I like both but I realize that they will only be in original condition once.