More people, more leagues, social media, pinball arcade app, barcades, culture of vintage gaming being hip for 18-35 year olds, general economic inflation can all be attributed to rising prices. NIB prices rose because stern had no competition then when they finally got it the established prices were on a high.
Right now they're just trying to beat each other in quality.... not price. 8k+ is really tough and tough to make money on a route but in order for the price to go down a chinese/korean/vietnamese manufacturer will have to come in and make a nice game for 4-5k to drive down prices in the NIB dept.