I suspect we'll soon be reaching a sort of saturation point with solid-body instruments. They've been in mass production now for more than 65 years, and unlike acoustic guitars and other things, they don't tend to naturally degrade past the point of unplayability --at least not on a time scale that has made itself known yet. Each year's production adds to the stockpile of instruments that are already out there on the market, and I don't think the number of people willing to spend money on a non-beginner instrument goes up by that amount every year. The reason prices have held steady or gone up, even for non-"vintage" stuff is that the average number of instruments owned by players keeps rising.
As a few people have mentioned, the generation now settling into retirement include a lot of people who collect guitars, and during the 1990s and 2000s big chains like Guitar Centre in the US actively encouraged that with marketing and finance deals. I know a few older folks in the US who own many more more instruments than me, despite never having learned to play. They bought them because they thought they were cool things (they are, in all fairness) and because they'd heard they'd increase in value.
The actuarial tables are starting to look threatening for a lot of those folks, and they're probably not going to ask to be buried with their 17 identical sunburst precision basses.