Prices aren't going down anytime soon. Pinball is in a sweet spot right now. retro while still be fun. Appealing to both young and old. Collecting and competing are both surging in popularity. Unlike old video games the new technology has not surpassed where we were in the 90's. Yes improvements have been made but those older machines were built rock solid compared to today's games, and with updated rule sets appearing more and more every day, the older machines will still stay relevant for years to come. And despite what many in this thread contend, the 80's and 90's kids aren't dying off anytime soon.
There is no shortage of guys (and gals....ok mostly guys ) around the world with plenty of money and desiring an awesome man-cave. They will continue to pump up the new market, as the new machines are a blast to play and many do not want any repair problems. The new machines will take a small hit in value, but they are certainly bringing up the value of the older machines as well.
Many of the collectors I know all started with one machine and somehow they fill every available space. There is a desperate shortage of supply in my area, and any machines that get listed are gone within minutes or hours if they are priced anywhere near pinside prices. Pinside prices are also one of the driving factors to prices going up. It takes a LOT of research to correctly price the market for any given machine right now. Pinside seems to be the go to price guide at the moment. And the faster the pinside prices rise the more it feeds the pinflation. Supply of the older machines isn't increasing, but demand sure is.