I am about to complete my Ph.D. (< 2 m.).
I had a very busy advisor, brilliant and intelligent, but acting as an ‘obstacle’ rather than an ‘incentive’ to complete the thesis (regrettably, a textbook case).
I got a rather good postdoc position at a US university (not frustrated ;) ).
Here are detailed some arguments (using some references from the web) why I think we SHOULD reduce the number of PhD offers and, as a result, the number of PhD students. The expected (positive) effect would be an increase in the quality of research, guidance, health, and career of those enrolling for a PhD. Ultimately, the money not used for PhD funding could also be redirected to senior researchers (postdoc, tenure-track) and help better balance the 'experience pyramid' of the research ecosystem.
Are there any good counter-arguments why we SHOULD NOT reduce the number of PhD offers?
- There is an oversupply of PhD positions.
Although a doctorate is designed as training for a job in academia, the number of PhD positions is unrelated to the number of job openings. [...] America produced more than 100,000 doctoral degrees between 2005 and 2009. In the same period there were just 16,000 new professorships. [...] In Canada [...] universities conferred 4,800 doctorate degrees in 2007 but hired just 2,616 new full-time professors .
According to figures from the Royal Society (UK), only 3.5% of science PhD graduates end up pursuing longterm careers in university research, and fewer than 0.5% eventually become professors .
Only a few fast-developing countries, such as Brazil and China, now seem short of PhDs .
- 1 is partly explained by the low cost of PhDs.
PhD students are cheap, highly motivated and disposable labour. With more PhD students they can do more research, and in some countries more teaching, with less money .
From the point of view of a lab head, for example, each graduate student is a source of cheap, clever labour that can do the grunt work needed to make the next discovery .
- Lack of guidance from supervisor (partly explained by 1).
I know of professors with 10+ PhDs 
The supervisor-supervisee relationship is generally awkward and confusing, and sometimes – maybe even often – uncomfortable and challenging. At my university there is no required mentorship training. Everyone has experience throughout their graduate and post-doctoral training, but in most cases, there is no formal course or body that guides staff in best practice. Most programmes that do exist are optional and ignored. 
- Students do not start PhD for the good reasons
In one study of British PhD graduates, about a third admitted that they were doing their doctorate partly to go on being a student, or put off job hunting. Nearly half of engineering students admitted to this. 
- A lot of PhD drop-out (partly explained by 3 and 4).
In America only 57% of doctoral students will have a PhD ten years after their first date of enrolment. In the humanities, where most students pay for their own PhDs, the figure is 49% .
72.9 per cent of the 11,625 students from the UK or the EU who began full-time doctorates in 2010-11 will obtain a degree within seven years.
Doctoral attrition rates remain high in North America, at an estimated 40% to 50% (Berelson, 1960; CGS, 2009; MERS, 2013; Nettles & Millett, 2006). However, they vary across disciplines, being higher in the arts, humanities, and social sciences and lower in the natural sciences (Bowen & Rudenstine, 1992; CGS, 2009; Elgar, 2003; Nettles & Millett, 2006). [...] Perceived competence appears to be the cornerstone of doctoral studies persistence (completion and dropout intentions) and is predicted mainly by autonomous and controlled regulations and advisor support.
- No economic and background advantage of PhD compared to Master’s
PhD courses are so specialised that university careers offices struggle to assist graduates looking for jobs, and supervisors tend to have little interest in students who are leaving academia. [...] Writing lab reports, giving academic presentations and conducting six-month literature reviews can be surprisingly unhelpful in a world where technical knowledge has to be assimilated quickly and presented simply to a wide audience. [...] PhDs in maths and computing, social sciences and languages earn no more than those with master’s degrees. The premium for a PhD is actually smaller than for a master’s degree in engineering and technology, architecture and education. Only in medicine, other sciences, and business and financial studies is it high enough to be worthwhile. Over all subjects, a PhD commands only a 3% premium over a master’s degree .
- PhDs face significant mental health issues (partly explained by 3 and 4).
Approximately one-third of Ph.D. students are at risk of having or developing a common psychiatric disorder like depression, a recent study reports. Although these results come from a small sample—3659 students at universities in Flanders, Belgium, 90% of whom were studying the sciences and social sciences—they are nonetheless an important addition to the growing literature about the prevalence of mental health issues in academia. [...] On the plus side, having an inspirational supervisor partially offset these risks. So did interest in an academic career, even among students who thought they had little chance of ultimately making it. 
 Why doing a PhD is often a waste of time https://medium.economist.com/why-doing-a-phd-is-often-a-waste-of-time-349206f9addb
 Too many PhDs, not enough tenured positions https://www.timeshighereducation.com/content/too-many-phds-not-enough-tenured-positions#survey-answer
 Too many science PhDs? Not if unis train them for careers outside academia https://www.theguardian.com/higher-education-network/blog/2014/sep/15/university-producing-too-many-science-phd
 The Scientific Century securing our future prosperity https://royalsociety.org/~/media/Royal_Society_Content/policy/publications/2010/4294970126.pdf
 Not all PhD supervisors are natural mentors – some need training https://www.theguardian.com/higher-education-network/2017/jul/28/not-all-phd-supervisors-are-natural-mentors-some-need-training
 How many PhD students are too many? How many PhD students are too many?
 PhD completion rates, 2013 https://www.timeshighereducation.com/news/phd-completion-rates-2013/2006040.article
 Dropout intentions in PhD studies: A comprehensive model based on interpersonal relationships and motivational resources http://selfdeterminationtheory.org/wp-content/uploads/2016/11/2015_Litalien_Guay_Contemp_Educ_Psych.pdf
 Ph.D. students face significant mental health challenges https://www.sciencemag.org/careers/2017/04/phd-students-face-significant-mental-health-challenges