We do still teach the 3 separate sciences where I work.....a state comp
I am currently teaching 45 children Biology at AS level. Of these all but a handful did double sciences. They have been 'setted' into B1 and B2. None of the top set did separste sciences. Not statistically significantly , since the n is too small, but for me it is interesting.
In my professional judgment having done double scineces is no handicap to these scientifically able children. As their teacher, I would far rather them have some breadth of study at GCSE. If you want to argue that the scince curriculum should be reorganised to give teachers more time to teach the kids scientific method, I'd agree with you. But simply having an extra GCSEs worth of scinece Knowledge stuffed into them, doesn't make them better scientists. It makes them better at remembering scince 'stuff', not the same thing at all. Or are you in favour of rote learning?
But what would I know DC eh? After all I only understand the sylabuses for all three types of examination, teach the work, and also work with the kids directly after they have finished their GCSEs. I am also a rather good scientist, with a significantly better than average understanding of my subject.
Oh, and I know what the kids are studying now following the changes in the science curruiculum. Bet you don't!