What I've always hated about that is there is a sense of an immutable judgement going forward if at some point you perform poorly. Surely it's obvious that that idea is merely an approximation and life is somewhat more complex?

Isn't it the case that if past performance is an indicator of future performance then there is no hope of improving anything?

I really think there is quite serious denial of the possibility of bad things happening to people which effects their performance and it not being their fault, often with very serious consequences for them.

I have a friend who had a family crisis occur during his A-levels; the impact on his future prospects was huge and he has had to struggle to stand a chance at getting a decent job. I personally had a family crisis during my degree which several impacted my performance and has destroyed my confidence, something which still impacts me today (though I did manage to get a 2:1 despite it all), does that mean I cannot perform well in the future?

I understand what you're saying about there being an overall correlation but I really do think you have to be careful about it - on the whole, it may be true, but it's the individual edge cases that matter, and adopting that position as if it was some immutable fact is dangerous.