Earlier quoted context omitted.
What does "senior and middle positions" mean? Are you talking about something that has the potential to be a tenured position, non-tenured academic positions, or non-academic positions?
By senior I mean "senior or associate professors and lecturers" like myself, and by middle I mean postdoc full lecturers with at least 5-10 years. I don't know what to say about the word "tenured" because it's meaning seems very different in the UK than it is used in USA. In the US I get the idea people think "tenure" makes you impossible fire. Yet here I know of two full professors with over 20 years service, scores…
I also knew plenty of more senior professors that left the field, which didn't encourage me to backfill it. Strangely enough, whatever difficulty there may be in the more senior ranks, entry sure seems like a grind.