The Problem of the Steady State
Assume that the number of permanent academic positions in a given field (e.g. astronomy) remains constant over time. If that is the case, each retirement (or other form of departure) from a permanent position will be replaced by one, presumably junior, scientist.
This means that over an academic career, on average, each academic will produce just one PhD who will get a permanent job. This of course doesn’t count students coming in from abroad, or those getting faculty positions abroad but in the case of the UK these are probably relatively small corrections.
Under the present supply of PhD studentships an academic can expect to get a PhD student at least once every three years or so. At a minimum, therefore, over a 30 year career one can expect to have ten PhD students. A great many supervisors have more PhD students than this, but this just makes the odds worse. The expectation is that only one of these will get a permanent job in the UK. The others (nine out of ten, according to my conservative estimate) above must either leave the field or the country to find permanent employment.
The arithmetic of this situation is a simple fact of life, but I’m not sure how many prospective PhD students are aware of it.