Estimation was always going to be fraught. Teachers aren't the universal unbiased body some think.
Indeed. But having acknowledged a different standard of assessment to other years, you don't then decide to change it to yet another different system...
Take, for instance, the nonsense of using the previous years' results at the school as a guide - schools can improve. Cohorts can be better (eg my cohort at my school did better, especially in league tables, than the cohorts either side of it - teachers don't suddenly get better then worse).
Then there's SATs results and the like - while seemingly confirming merit, some people are late bloomers, others are not. And aren't they ranking people in their year groups by these past results? So, for science, having come what was almost certainly top of the year in the level 5-7 paper at KS3, would either be demoted in the pecking order because I chose to stay at primary school to the end rather than leave a year early (those who formed the first year 7 class for years at my secondary school got to sit the level 8 paper, which the rest of us weren't - so others would have level 8s), or my dropping down from top of the year to a middle of top-set would be ignored and people would be denied A*s because I was only A-grade in Chemistry and Biology. Likewise, I came bottom in English in year 9, so my hard-worked for C-grade might have been dropped to a D-grade (which I think 1 or 2 people got) due to the rankings. Either way, it's a distortion that make teacher-given grades look sensible.
A massive mess was made when it was decided to use a different standard of assessment. The last two weeks (and I'm sure next week too with GCSEs) have been exercises in exacerbating the mess.
the opportunity to retake
Not picking on you, but the wider conversion. 'take', not 'retake'. 'sit' not 'resit'. It's not a second chance, but the first attempt at the exam.