I only know the european situation:
The current generation >40 grew up with pinball machines in arcades, cinemas, pubs and even local village fairs and can now afford to buy some.
But what about the next generation? Most don't even know pinball machines, cause they are rarely to find compared to 20 years ago.
So the value of machines climbs as long as the current generation is interested in owning or collecting some.
Like lots of hobbies we don't know yet, if our kids or the next generation in general think these machines are still "cool" or "trendy" or if they think its old stuff from the old generation. In that case the value would decrease rapidly.