I would agree with you that the ammount of pinball machines currently in existance is expanding. HOWEVER. I also believe that the rate of pinball enthusiasts is growing faster than marketplace is introducing them into the market. Supply and demand is keeping up with pins in the 5k price range, but new entrants are causing the rise in prices of pretty much all the lower end Solid State machines.

Also, I would put the number of pins of any given variety in existance at no less than 3 times what is listed in people's collections on pinside.

Case in point. There are 36 Primus pins in pinsiders collections. We know exactly 100 were made.

Theory holds that:
1. Pinball nerds are more likely to own this machine, register it, and have it on pinside.
2. Not everyone on pinside lists personal games in their collection.
3. If 1/3 of one of the most likely pins to be registered are listed on pinside... logic dictates tripling any number of pins on pinside would give a more likely correct number for existing number of any type of pin.

Also relevant.

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If you really want to know you can purchase a report on the market... for $2,999.

https://www.syndicatemarketresearch.com/market-analysis/pinball-machines-market.html