"Bit that always bemuses me though, is why they don't just embrace the Windows PC model (or probably more accurately the Windows PC laptop model) - with a Linux twist."
It's the chipset manufacturers that are blocking that plan. Many of them design SoCs and specialized chips as highly-competitive black boxes (because they don't want to Give Information to the Enemy). Unlike in the PC world where most stuff was based on discrete standards, a lot of ARM-based hardware relies on proprietary arrangements covered in trade secrets and NDAs. Some manufacturers are more forthcoming, others aren't (some like Allwinner even violate the GPL it seems but don't care because they have connections).