Upside down is not the right term. It means to still owe more money on something than it is worth in total. Some people walk away from houses (despite the hit to their credit rating) because of this. A lot of people buy gap insurance when in this situation, so they don't still owe money on a car they don't even have if it is stolen or totalled in an accident (lenders may insist on the buyer carrying gap insurance). This situation doesn't really apply to pinball.

Pinball people hate car analogies, but minting out a lesser game is like restoring an undesireable car. You do so because you love it (the pin or the car) and you want it in mint condition. In both cases it does not make financial sense, because it will never be worth the amount of money you have into it. I like Ted's suggestion that it should be called a "keeper" because it will never make financial sense for you to sell it at a loss. Then whenever you die your inheritors will sell it for whatever they can get, because they won't have the appreciation of it that you did.