So, @Conmoenotcalltoe, am I right in thinking that, under your system, some pupils will just do GCSE maths, English and the three sciences, but others can choose to do more subjects to GCSE. In that case, how will universities and employers choose fairly between the lot with just 5 GCSEs, and the ones who have 8 or 9, for example?
And how will we encourage people to do subjects outside your basic 5, if kids who have zero interest or ability in science subjects still have to do all three? I was that kid - I was supposed to do O level physics and 16+ chemistry (16+ was the precursor to GCEs and then GCSEs - you got either a CSE grade or an O level), alongside maths, English language and English literature, history, French, German and music. But in the run up to the mocks, I became really stressed, and the worst thing was that, try as I might, I simply could not get a grip on physics, and that impacted everything else. In the end, the school let me drop physics, and I went on to get 5 As and 3 Bs in my other subjects.
If I had been forced to do all three sciences as well as maths and English, I would have failed physics and it would have impacted my other results too. Plus having to add in biology would have meant I would have had to drop one of the subjects I was actually good at - I was much better at the humanities and music.
Your one size fits all approach will not fit all pupils.