Actually I would suggest that we're probably at a fairly high point in specialist publishing. If one takes a look at the numbers, quality and variety of books, magazines and booklets that are available, I don't think there's been a better time. From a reader's viewpoint I think we're at a good stage. One only has to pick-up a book from maybe thirty years ago to see how much better most material now is by comparison (I'm generalising here of course, as there are exceptions).
The problem is for the publisher, as I've explained previously. In the book world, specialist publications do quite well now because they can be produced in small quantities (maybe only a thousand or less in many cases) by small companies - often no more than one-man concerns. They rely on authors who are often retired pilots, industry men, etc., who are happy to write material almost as a hobby, so they don't really care if they only make a few bob for their troubles. This enables these little publishers to survive.
It's nice for readers while the bubble doesn't burst, but one has to wonder what will happen when all these authors have either died, or have written their once-only "tour de force" and have nothing more to offer. What happens then? There is no other source of new material. Worse still, all the professional aerospace authors will all have gone, as they can't survive by writing for specialist publishers who can only pay them peanuts.
Similar things are happening with magazines. Things are okay at the moment, but the respected, professional writers are slowly disappearing (many are already long gone) because they have to make a living, and they simply can't do this by writing for magazine and book publishers any more. Consequently there is undoubtedly a gradual "dumbing-down" of material, and it's a trend that seems difficult (if not impossible) to reverse.
In some respects the rise of the internet has been quite poisonous - and its effects will become even more evident as the years go by. It's just that at present most people see only the benefits.