I bought 18 machines the were not NIB. Of those 3 were listed on the internet. While the sample size is too small to be of great statistical value I would say take the number of machines listed and multiply by 5, which gives you a rough estimate of what's still out there.

I also noticed that more popular, older titles don't come up for sale as often. This probably has 2 main reasons:
1. Popular games tend to have longer hold times by their owners while boring games are sold more frequently.
2. Popular games where played to death when they were new back in the 70s and 80s. Then parted out and junked.
For example Joker Poker (1978) had 10K games made but it is difficult to find these days. Given the electrical problems many early SS Gottliebs had they probably didn't stay around very long and were dead after a few years.
Seawitch (1980) had 2503 games made and is extremely hard to find.
The popular TOTAN, AFM, MB, ... had 3+K games made and comes up for sale very frequently. By the mid 90s games were better quality and weren't played as much anymore. That's why, based on production numbers, relatively many 90s titles are still around.

Since the early 2000s you have serious collectors. In the 70s and 80s very few people collected machines. I think most of those were lost due to poor maintenance, improper storage or as parts donor. I played a lot of pinball in the late 70s and early 80s and I didn't know a single person that had a machine in their home. Today I personally know about a dozen people each having 5 - 30 machines at their home.
Back in the old days (1975 -85) there was an arcade at every corner packed full of pinball. Once those machines left the location they vanished. The vast majority certainly didn't end up in peoples homes. I have the feeling nothing good happened to them after they left the arcade. Storage is expensive and if nobody pays any real money for it, it gets parted out and the rest gets junked.
1979 was the year with the world wide highest production of machines, yet they come up for sale relatively seldom.

About 247000 machines were produced in 1979 alone versus an estimated 50000 in the 6 years from 2010 to 2015. Unfortunately the production number for the recent Stern machines are only available for some models so I had to take a very generous guess. Still you see 2010 -15 models up for sale more frequently than 1979 models, that's my point.