You cant blame them. There is no real reason these are commanding such high prices. We all love them but nobody can justify a 200k 1968 SUV so this market will settle eventually as they all do. When a Nicely done Bronco brings more than a rare original Boss 429 Mustang you have think bubble.
Boss 429s are now getting $240K so your theory doesn't really work. I can tell you building my own Bronco to the specs some of these Broncos are at I'm into it for between $80-90K in purchasing the Bronco and parts/paint. The vast majority of labor is done by me. Figure about 1000 hours at an average of $50/hr and you're tacking on another $50,000.
https://www.classic.com/m/ford/mustang/1st-gen/boss-429/
That, and while you see a concentration of high end Broncos at BJ, in reality there's really not that many in a country of 320 million people. One of our customers is a Ford dealer. The owner bought 2 at @$200,000. Say there's 250 high end Broncos. That's 5 per state average. I know one guy who bought 2 in my state. The real issue is people trying to sell $50,000 Broncos for $150,000. They're not getting their $150,000. If you track BringaTrailer and see which sell and which never hit their reserve, you get a better feel for what's really happening in the market. Is there a "bubble"? Maybe because of the new Bronco, but I've been here since 2011 and I've been hearing about a bubble since I joined.
Page down to see the graph:
https://bringatrailer.com/ford/bronco/
Meanwhile think about all the companies who are selling them. Gateway, Velocity, Nick's Trix. They're all busy. There were 230,000 Bronco's built 1966-1977. There were 560,000 mustangs built in 1966 alone. So there are less than half the number of all early Broncos as there are just 1966 Mustangs.