Don't really have a clue what either of you are on about, so I'll try to make my original point again.
Forget a restricted market, assume it's unrestricted. And forget PH and forget a mixed HC fleet.
Say that over 20 years the number of HCs remains static at 100 vehicles. It doesn't move. There were 100 vehicles at the start, and 100 vehicles at the end.
Your thinking seems to say that in such a situation no plates are ever handed in. But that's not the case. Some will leave the trade while others start from scratch.
Some will retire, some will move onto another job or business. Some will be part-time anyway, and/or just doing it as a stop-gap until they get a 'proper' job. Some will find that they just don't like the job, or they grin and bear it for a few years and then get fed up. Some may well not be able to make the repayments on their cab and it's reposessed or they hand it back, or whatever.
Anyway, five vehicles giving up per year doesn't seem unrealistic. Probably a few more, but five is a nice round number. And five new starts per year. So overall the two cancel each other out, and the fleet stays at 100.
But with five vehicles per year leaving the trade, over 20 years that's 100 plates handed back
Sounds awful when it's spun in a particular way, but in reality it's no big deal at all. People come and go, end of.
Of course, some may well have gone 'bankrupt' - whatever that means exactly - and that's unfortunate, but it's not necessarily representative of the bigger picture.
or 