The two tier system, O levels and CSEs, was phased out in the early 1980s, replaced, initially, with the 16+ then GCSEs.
Using separate exam methods platforms according to ability creates a discriminatory system. Obvious Unis will prefer the higher qualification but schools may choose to limit provision to the system that best suits their demographic.
Pre 1980s you would not be considered for A levels without O levels. The curriculums were continuous so if you hadn’t taken the O level it was a struggle to continue in STEM subjects. There were some subjects not offered at O level ( business studies, economics etc) that were easier to take up at A level.
Universities would probably welcome the return of old style O & A levels, it was much easier to identify the more able candidates. Not necessarily fare if you struggled with exams but as mumsnetters are always telling us a clever child will succeed regardless of environment.
I agree that it was a better system but it wasn’t particularly a fare system.