> One answer is that the private-equity industry is devouring them.
Another is that public companies buy them and they consolidate. Another is management buyouts. Another is it's harder to run a business than it used to be, for better or worse, so they close.
> One-fifth of the market has been made effectively invisible to investors, the media, and regulators.
No. You still have financial regulations to follow. If you're a medical device company, you'll still have to follow those regulations. You have employment regulations. All sorts of regulations.
You just don't have to follow regulations for public companies (such as "don't talk about this thing with anyone outside a list until you've announced it publically, because that would be unfair to other investors), which are expensive and onerous, nor follow the whims of the market, which can be expensive and onerous.
E.g. Dell went private and the quality got a lot better.