maybe they CAN achieve the grade 4 and improve their long term job/college prospects
This seems like a reasonable argument, however only 26.9% of resitters passed English and 29.5% passed maths GCSE. A huge amount of time, effort and money and the vast majority still failed.
Es and below could sit functional skills.
GCSE resit classes tend to be a couple of hours a week alongside the students' main courses. Trying to get them to pass in about half the number of hours what they couldn't manage the year previously while they are also having to cope with what sixth form throws at them is quite a big ask. They finish their exams in June, spend the next couple of months forgetting all their maths, then have to remember it all again. The lessons are just about enough to stop them getting worse and any homework tends to be put to the bottom of the pile under their A-level load/BTEC coursework.