"The real ambition for anyone with a Ph.D is to land a tenure-track position, or at least a researching position"

I agree that those are important factors. But I also think of an English teacher of mine in high school who worked on and got a PhD, and stayed as a teacher. As for me, I am thinking (I'm in my 40s) to go grad school because there is a specific research topic I want to work on, concerning to the historical development of the field I've been working in for the last 15 years, and there are a couple of university departments which specialize in the topic, which would give me better access to historical materials and better networking.

For what it's worth, I develop software for pharmaceutical research, where most of the jobs are in industry, and most of my users have a PhD in chemistry. Which means that you're right that for most PhDs I know, the PhD is a union card of sorts for a job in their field of interest. On the other hand, that's not always the end goal. A number of pharmaceutical and biotech companies are run by PhDs, who switched from research to management.

"Increasing research in the humanities would not be nearly as useful for society"

This is only meaningful in the context where the government funds the research, right? If someone funds their own PhD program then why should society care?

"exactly what I meant by #2, at least in part"

Ahh, yes. I was thinking about how the increasing number of undergrads would lead to more adjunct positions, but likely not significantly improve their wages. Adjuncts don't tend to teach grad students. But you are right; wait about 30 years for the current generation of PhDs to diminish, and there will be a smaller supply.

"the exact same way antitrust laws are anti-free-market"

Exactly! That's why I don't think supply and demand is 'simple', as you called it; there are a lot of ways to tweak the supply and demand, and the market will respond to those new conditions.

For me, limbo is a worse state than success or failure, so I think an earlier final decision is better. But I can't speak on that for all.

(Also, I would prefer national health care, which would also help the adjunct situation.)

"I'm skeptical that unions will help"

Oh, I am as well. Unions have been pretty well defanged in the US, with few exceptions. But it's at least as reasonable as your #1. :)

I don't think you should consider an adjunct position as "breaking into" the field of teaching. It is teaching. Some of the adjuncts have more teaching experience and are better teachers than the professors, and get paid far less.

You structure it as "volunteering", but that could be said of almost any position. Mine workers, if paid less, are volunteering their time to the company, no? Lilly Ledbetter, who was paid less than her male co-workers for decades, was really just volunteering to work at a lower rate. That's a stretch; she specifically didn't know what her co-workers were being paid, but in other positions it used to be explicit that men were paid more than women for the same job. So were the women volunteering their extra work to the company?

As a member of several organizations, including the Python Software Foundation (a multi-million dollar non-profit), I know quite a few people who are part-time bureaucrats for no pay.