My understanding of arts and humanities PhDs is that they are largely self-funded, much fewer in number, and mostly vocational for the truly dedicated.

I think you might have a different perspective if you were to do a science PhD. It's exactly what is described. It can feel like you are low-wage labour. You are low-wage labour. From the academic group leads' point of view, your labour is "free" because you come into their lab with funding from a national funding body or charitable organisation (e.g. BBSRC, EPSRC, MRC, Wellcome Trust, Leverhulme Trust, Cancer Research UK etc.) and so you don't cost them a penny of their grant money other than in lab material costs, and you even get an allowance for that in your funding. Actual employed staff (Research Assistants, Research Technicians, Postdoctoral Researchers) cost them a full time salary on their grant applications.

There's a very wide pyramid structure, with PhD students being the base, topped by a much smaller number of Postdoctoral researchers, with a very small number of career academics on top. This imbalance is bad for several reasons. Firstly, the prospects for an academic career are dire. Most people leave after their PhD. Most of those that remain leave after one or two Postdocs. Those that remain attempt to climb the academic career ladder, but most of them will also fall off it, due to funding issues, lack of research success or the sheer stress and uncertainty of it all. Secondly, the glut of poorly-paid PhDs has eliminated entire categories of career positions. Research Technicians and Research Associates used to be good positions in organisations with long-term prospects. They are now almost nonexistent, and these people used to have huge amounts of knowledge which is now lost whenever the current PhD or Postdoc moves on. Postdoc positions are also a position which should not exist; it was a transition between training (PhD) and an academic position. It's now a decade-long limbo with poor prospects for most. In previous decades, existing academics could go straight into a fellowship.

If you spend four years of your early career doing a PhD, that's four years you get paid a minimal wage, work insane hours, for precious little to show. The chances of success are strongly weighted against you. That's four years you could be getting paid a proper salary, getting useful experience and being promoted, getting and paying off a mortgage, into a pension, starting a family etc. I went into one without considering all that. In hindsight, I enjoyed the research work and the environment for the most part, but it was strategically and financially a net negative, and I would struggle to recommend it to anyone. The numbers simply do not add up, and having a PhD doesn't give you better job prospects except for a few very niche roles. Experience, however, does open many more doors.

There are far, far too many PhD students. They could halve or quarter the number and make up the shortfall with permanent staff positions. Same for Postdocs. They could also use the saved funding to be able to fund grant applications without rejecting those which pay for qualified staff rather than unpaid students. That's a vicious circle which leads to systemic underestimation of the true cost of research work. The whole system is fundamentally not fit for purpose. There are too many people applying for limited amounts of funding, and this is the result of a race to the bottom since it's essentially cost-saving on salaries by employing cheap inexperienced labour through selling them a pipe dream.

Am I slightly embittered by the experience? Maybe a little. I now work in embedded programming for medical imaging and medical diagnostics. So the experience I gained did help a little. But it wasn't essential. I could have got there quite easily without it.