Most private schools in Scotland already do GCSEs and A Levels instead of Scottish qualifications.
No they don't. A couple of the VERY expensive schools like Fettes or Gordonstoun offer A-levels or International Baccalaureate, probably in recognition of the fact that most of their students are from England or overseas. Your George Watsons, Heriots, High School of Glasgow, Dollar Academy, Hutcheson's Grammar, Lomond School - all do Nat 5s and Highers, and Advanced Highers.
For my kids with their university educated parents who are motivated and interested in supporting their learning, there is a lot to be said about rushing through S3 or S4 material as quickly as possible, leaving more time for teaching to the Higher exams in S5. Ever since I was at school in the dates of O-Grades there was always this concern that the level jump from Nat 5 / Standard Grade / O-Grade up to Higher was a LOT and if you could tweak the curriculum to start teaching the Higher course in January rather than after the exams in May, it might make S5 all a bit less manic.
But then you have kids who will not be doing Highers for whatever reason. They NEED their Nat 5s to get into college or onto an apprenticeship. Even if they ARE doing Highers many courses require at least a B in a Nat 5 subject - DS is looking at business studies degrees which need a B in Nat 5 maths at least (if you don't have the Higher).
This proposal is clearly nonsense though, but the SNP have form for making rubbish decisions. Ferries, anyone? Motorhomes?