I'll try to address some of your points.
"If based on teachers' knowledge of the student, it would suggest that, unless you are very unhappy or in the wrong school for your ability, it would be better to stay where you are and where teachers know you." Ds's school does not have a sixth form so he could not stay on.
Scrapping the personal statement isn't happening, it is an amendment to the free form format. Ds1 is already at uni, he was predicted and got 4 A stars, high grade GCSEs and so the personal statement is where he could talk about his love for his subject which for him started back in year 10. If there is no personal statement and he applies to a course that has a 5 to 1 application rate, how does the uni choose who to offer to if they all have the same grades? 8s and 9s are considered the same for uni. Ds was an early entry (Oxbridge) and so his UCAS predicted grades were based on year 12. Deadline is 15th October, his application was submitted at the start of October. There is no time to test again before that application goes in.
Right from the start of year 7 DC's school told them, do your best, more doors open, more opportunities are available and we want you to have choice. Basically that their job is school and learning and achieving the best for themselves. This ramps up when they get to year 9. Year 12 we had parents' welcome evening, same speech was given to the students in assembly. This year matters, why it matters. The college provides support whether that is uni applications or jobs/apprenticeships. They have future focus mornings, Ds2 like Ds1 is also on a competitive entry track ie predicted high grades for A levels. They are continuously told year 12 is important. They even have intervention if you are not doing enough work, they support the children to complete the work rather than negative consequences.
Contextual offers are for children who are in care, have been in care, are a carer, disabled, on free school meals, first to go to uni or live in an area where moving on to higher education is low. It is usually a 1 grade drop or the very most 2 but the uni has to believe they are capable of doing well on their course. Not all unis offer it and if the uni does offer it, not all courses at that uni offer it. It is meant to level out the playing field.
"Yes of course but if you're predicted e.g. BBB, you can't apply to a course that requires AAB. Perhaps you SHOULD be able to apply to whatever and then, once the results are in, if you manage to hit the grades you should be accepted." But that doesn't take into account the applicants who were predicted AAB or higher and did achieve those grades. There are only so many places on courses. Entry grades are the lowest they will take someone in on, so AAB entry will see applicants with A star A star A applying. The UCAS process is a legal contract, if you get the grades specified on our offer we will accept you onto our course. Hence the massive scramble in 2020 when there were far more applicants with the specified grade than spaces on the course, plus first year accommodation. Usually some are culled due to the exam grade curve being applied.
@Phineyj So good to come across an economics teacher. I agree post exam applications would be better but logistically I don't know how that would work without a massive shift in marking, results and uni start dates. My son took economics as a new subject for year 12 and absolutely loves it. He is applying to uni probably as a pure economics course which he knows is incredibly popular. He is currently debating the whole BA/BSc course options as he is studying maths and further maths A level but considering BA due to the unis offering it.