I don't think it has much to do with nostalgia, but you can't deny that pinball is cool and fashionable right now.

And like all cool and fashionable things it will become uncool and unfashionable at some point. And when it does you'll have a bunch of people with machines they don't play much and don't know how to fix taking up space in their basement.

Then the question becomes how much longer is it going to be cool? I don't think it's on the downwards spiral but I'd think it's coming in the next 2 years (no data or anything to back this up, just my hunch).

Another thing is that people probably work their way up, starting with less popular titles then moving on to the A titles. Those people in the know and many years in the hobby will be the first ones to see price trend flattening but won't be able to sell to the more mainstream crowd who would never consider buying a 10k machine so the tower will topple at the top first.

(Or maybe not! Not losing sleep over this, but seems like a likely scenario to me... )

Edit: I watched The Big Short this weekend too, so maybe I'm seeing bubbles everywhere

Is pinball trendy? I think when PBR has its own machine made, you can say it is. You see it popping up in TV ads, barcades popping up too, etc. Does trend affect prices? I would think it does, a lot.
Is the trend going to taper off at some point? Yes it will, like all trends.

However, pinball has an intrinsic value that a lot of trends don't. They're not Beanie Babies, they'll never be worth 0. They're fun to play (most of them are), nice to look at (most of them are), they'll always have that americana appeal (most of them will), and there's a new generation that is being turned on to them.

From being the coolest thing in the world in the 70's, pinball hit a huge slump in the late 90's, and now it's creeping back up again to near 70's level of popularity.
The best A and B titles are not going back to 2000$, but I would think when the trend tapers off, the market will get a correction, and I think at some point you might have a hard time selling off those Super LEs for anything close you got them for, and some of those crappier DMDs that are selling at crazy prices (I've seen SFII's advertised for >2k over here) may go back to eyesores sitting unplayed and unsold in basements at some point, while the good machines retain good value to the collector core which will be unaffected by the trends.

(I like to do disclaimers in these controversial topics: I've been in the hobby for about 6 years, was I affected by said trend, probably to some extent, and I think the PBR machine looks cool as hell )

Yeah I guess you're right, that was a gross overstatement on my part. There are no monocultures anymore and I don't think it can ever get to that level again, just as no band will ever be as big as the Beatles again...
That being said, my point was that some people are saying the market's been steadily going up for 20 years as support that it will keep growing. That's saying it's been growing since the last time it recessed, that doesn't mean it's going to keep growing forever.

That being said, again I don't see a plummet because of the intrinsic value of pinball, but there will certainly be a correction at some point in the future and a recession when the pinball trend has come and gone where only the cream of the crop games keep most of their value and the rest have a harder time finding buyers.
Now is that a good or bad thing, mostly good for most collectors I would think, just stating what I would think is a plausible scenario.

I think that scenario exists in some kind of feedback loop.
As the prices keep climbing, some people who put x money in a collection that is now worth let's say 5 times as much start cracking and selling off, market gets flooded and prices drop a certain amount.
Now it's not like there's a clear threshold at which that happens and everybody sells at the same time, which probably means some correction in the future is likelier than a plummet.
Again, if this was a pure fad with no intrinsic value the plummet scenario would be very likely, but although rather trendy pinball has a pretty great an staying inherent value and I think it's much better protected from high volatility.

You might not, and people who do might not want to "cash out" and sell everything, but as prices go up so does the pressure to sell some machines which may or may not cause some flooding of the market and act as a feedback loop to the price inflation.