It makes sense to have separate papers in some subjects - e.g. languages usually have reading/writing, speaking, and listening. And I agree a single exam doesn't help if someone has a bad day.
When GCSEs were introduced (I was I the first year, 1988,) part of the aim was to make them fairer by including coursework - it recognised that not everyone is well-suited to exams and a one-off performance.
Part of the issue is that GCSEs are very important to league tables etc. Every school can tell you its percentage of students achieving at least 5 GCSEs at grade 7 or above. But that completely overlooks the schools where getting someone from a 3 to a 5 is a far greater achievement than a very capable student getting straight 9s.
Also, we narrow the curriculum too soon. I resented being allowed to take only 3 subjects at A-level - I was an all-rounder. I wasn't really ready to give up maths and sciences. Specialisation does help some people who have clear preferences, but there are millions of 16yos who don't really know what they want to do yet. There is the option of doing the Bac, but that's not available at all schools. Perhaps that's not an issue in cities, but in places with large rural catchment areas and a reliance on school buses, you don't have those choices. And I can understand why Sunak wanted everyone to do dome sort of maths up to 18, but constant GCSE resits isn't the answer.
When I went into 6th form, we had a lot of people staying on for a year (when they could have left at 16,) doing things like extra numeracy and literacy. At the other end of the scale, they were supporting those of us who were Oxbridge candidates. I fear those less able students would not get the same options now. Although most people will now have to stay in school until 18 rather than 16, the focus is now very much on what counts towards league tables, which will work fine for those who are academically able, (I'm sure I'm not the only one who gets some enjoyment from the challenge of having to answer a question within a certain time,) but people aren't all the same and some would do better with other options. Options which mostly don't really exist.
But I don't really know enough of other countries' education systems to compare and contrast.