I think the most accurate way to think about the situation is to look at that of the very old electro-mechanical arcade machines. There are still some who collect them, but most of those deteriorated to beyond repair states and/or have long been landfilled. The rest are now either in museums, or personal museums. Sadly, most who are interested in video arcade preservation, really could care less, even though we might have played some of them when we were very young.
There may still be a market for original video arcade machines 20 years from now, but those who grew up putting quarters in them will very likely be gone, or so few in numbers, there will be little demand. And unlike the EM machines, the games themselves will very likely still be with us, being playable on your toaster (/s), further reducing demand.
With this in mind, I feel that the hayday for original arcade machines has already passed by, unless it is so rare and in such good
original condition that it will one day find it's way into a museum. I'm not saying that there isn't still a market, but if I had one I was trying to sell, I would probably consider decent offers, even if lower than what I wanted to get from it.
Pinball machines, on the other hand....