There are plenty of opinions here and plenty of folks with insight, but don't think anyone here as a crystal ball that can predict what will happen and even the best economists are often wrong. What I do know, it is a great time in pinball, and certainly we are in a better place than where we were in 2000, so right now I just plan to enjoy it.
I can list stuff like . .
Saudi-Iran sabre rattling
N Korea Nuclear threats
Global Terrorism
Oil prices keep falling
Fed raising rates
Election year uncertainty
Potentially shaky 4th q earnings
Uncertainty about China growth
And it certainly creates concern about how these things can impact the world, but we can certainly go back in time and find similar lists where the world did not implode (of course it could, but pinball won't really matter very much if that happens). Regardless, while the economy (both US and world), can impact pinball, the reality is we haven not seen that to be the major driver in pinball, so I think the conversation of Stern and other pinball manufactures is more tied to the economics within the sub-culture.
There is general agreement that we have seen growth due to new players/collectors joining our community over the last several years, and the question is "will that continue?". If we don't see new enthusiasts continue to join us, then prices for used machines will go down (yes, supply and demand) which will then impact NIB purchases, which will shrink that market. Right now Stern pretty much owns the market, and while they have lost a small bit of market share to JJP, Spooky, Heighway, etc. The growth of the market has more than made up for the share lost. However, if we don't see continued growth of players, that will change and as the market gets smaller, we will certainly see these manufacturers respond to that. Stern has not seen a drop in sales, yet, but if they do, then things could get interesting.
Ultimately there is only so much money in the marketplace, getting new players/collectors is key to the long term survivability, regardless of the economy. We have seen that over the last dozen years, the question is will that continue.
Also, each collector has only so much space for machines, so even if you have the money and the economy is good, does not mean a collector will buy more machines. I could afford more machines, but I'm only willing to dedicate X amount of space for pins.