I think that is due in part because buyers of those "boutique" brands tend to hardcore enough to participate on a continual basis in a forum like this one. I have always bought "off brand" folders/non folders, and I've observed that my level of dissatisfaction doesn't seem much if any higher than those who spent 3-10 times as much.
Well, from your own words you seem to look at the negatives, if not focus on them "...my level of dissatisfaction doesn't seem much if any higher than those who spent 3-10 times as much..."
Personally, I try to examine things not from a negative or positive perspective, but as an engineer what gets my dollar is what is engineered the best. It has served me well when buying two cars, a house, and a handful of bikes, including two folding bikes. The reason why Brompton and Dahon gets a lot of press has to do with the brands' level of engineering, if they were bad bikes by design or had poor build quality they wouldn't sell and they wouldn't have such a large fan base. That doesn't mean that there aren't other good folding bikes out there.
I only paid $50 for my first folding bike, a KHS-F20A (this particular bike I did not buy for the engineering but as a low-cost entry into the folding genre to find out what it is all about), but the KHS isn't nearly as good, or provide as much utility as my Brompton, which I ride daily. The KHS served its purpose, it taught me a lot about folding bikes from which I could make a more knowledgeable decision in the future. I wouldn't pay $50 for another one, though, and the model has been discontinued. The Brompton, however, is worth every penny.