All pricing is supply and demand. Buyers set what the market will pay. Sellers don't, they can list a price but who knows if that sale will ever happen.
The pandemic response cut pinball supply and inflated demand for stay-at-home big-ticket items. Some of the demand is permanent; I'm part of that; my job went remote permanently and so I moved out of the city to a big house with room to start buying machines. That's what the thread is saying, there's more high end demand but high end supply hasn't changed.
Supply and demand for ordinary machines will revert to the mean. There's already a good amount of recently-produced machines like AIQ, Mando, Rush in stock at distributors. Stern is catching up over time. GZ's big backlog is an outlier for that particular title, not a new norm.
Older titles aren't going to revert in price, as long as demand keeps escalating with new buyers while supply never will. The likes of Addams, TZ, TSPP will never go down by any significant margin.