World economies are interconnected. If the BRIC nations sneeze, the USA can certainly catch a cold. It's my personal belief that the global economy never really healed from 2008 (or even 2000 honestly), and that there's a lot of pain coming to the markets that have skyrocketed the last 5-7 years.
Pinball machines are collectibles. Collectible markets don't 100% mirror the economy as a whole. The hobby has seen a huge influx of collectors. It would take a big percentage of the new collectors to back out of the market to effect prices significantly. What could force those collectors out of the hobby? Possibly a recession where a percentage of the collectors lose their jobs and lack significant savings to ride out the storm. But the games are owned for the most part free and clear and not on credit so your typical pinball owner is probably more the cash-only and 'save for a rainy day' type. Most people in the hobby also don't own the game because of perceived future value, rather they buy the games to play so the impetus to sell when prices drop is only for people who bought them with price as the primary factor.
I would be more worried about a freeze in new game buying vs used prices. That would destroy Stern, JJP, Spooky, Heighway. The last recession did not see a used pinball price fall, but it did see Stern significantly alter their games for cost, divide buildouts by 'Pro/Premium/LE' etc, and force Stern to take on an investor. The NIB manufacturers may not be able to survive a halving of their sales for a year.