There are many threads about the "bubble" that some people see in pinball. All in all I see this rising market as a cycle rather than a bubble. I agree that it won't last forever, but I think it will wane rather than pop.

It's interesting to see that economists seem to argue the same points, and reach the same conclusions that those in this thread do. I think most Pinsiders who see the current market as a bubble are applying the "Greater Fool Theory." Maybe they're right, but only time will tell. http://en.wikipedia.org/wiki/Greater_fool_theory

One idea about bubbles is that they occur when people start to pay above the intrinsic value for an item. With collector cars, people began paying much more than a new car with greater technology cost. In addition replicas were made for parts, and even for full cars, that could be bought for much less than people were paying for the originals.
It seems to me that pinball at large doesn't yet have this problem. The vast majority of used games can be bought for less than half the price of a new game with similar features. And as we've seen, building a replica machine can be even more expensive than building a machine brand new. Even replicas of unique parts are very expensive to make, which is why we see so few reproductions of ramps or other custom toys. When it becomes normal for prices of used machines to be more than new machines, we might be in trouble.

Interesting reading:
http://en.wikipedia.org/wiki/Economic_bubble