Presumably this will lead to grade inflation.
https://www.bbc.co.uk/news/uk-53810655
See also Wales seeming being the first to switch to this approach: https://www.bbc.co.uk/news/uk-wales-53807854
Children/adults in care (people without families that can support them) might not be eligible for support for another year, and will often be expected to go out and work to support themselves. Poor families often rely on the extra financial support that ends with "finishing" education.
There are a lot of stories out there of children/young adults in exactly this situtation that should be off to skilled work training or university that might end up having their entire education wrecked over this despite being able academically. And this is all with the context that private school students often have not been subject to any "standardisation" process due to their small class sizes, or have had their awards upgraded above predicted grades.
We just need to give this year a free pass in much the same way as workers have been given a free pass with the furlough scheme. We can live with one year of inflated results.
So we'd need exams tailored to just the part of the curriculum that the students have actually covered -- which would be different for each school, making any kind of standardisation impossible.
Another thing people don't seem to be considering is the knock-on effect on universities, which -- if students who didn't complete the A-level course are given a hypothetical grade, whether based on teachers' predictions or modified exams or "mocks" or whatever -- will get an intake of students who are less well prepared than normal, and will need some kind of remedial support if they're not going to flounder in their degree courses.
Just giving students generous grades now, to calm the current storm, means storing up trouble for the future when university teaching staff are faced with a cohort that includes students who shouldn't be there and can't cope -- but their teachers gave over-generous estimates -- or who in principle should be OK but arrived woefully under-prepared because they missed a bunch of key A-level content.
If one student does not know key parts of the curriculum and the other does, then this should be reflected in the grade no matter what caused that difference, whether one of them got more schooling or studied on their own or took some out-of-school tutoring.
And if a diligent student who got all A's before missed out on key parts of the curriculum that's required for further study, then the final grade should reflect that they do not (yet) know the subject as expected and are going to need remedial support, i.e. their final grade should not be an A, the exam should ask for the expected content (no matter if it was not covered in school due to Covid) and the exam results and grade should illustrate whether the students have learned it otherwise or if there's a gap.
While that'd result in grades that would be far more meaningful than any of what's currently being done -- and universities, employers, etc would just have to take account of the special circumstances surrounding 2020 results when making admission/employment decisions -- I don't think it's politically tenable.