> It didn't prohibit innovation and optimisation on PC
And PCs don't have their architecture determined by legislation, so that further proves the point.
> Having half a dozen standard phone sizes seems reasonable, along with standard sockets for CPU, GPU, memory etc
There are a lot more possible sizes or form factors, CPU and GPU aren't necessarily separate, and memory is typically either soldered-down chips or built into one of the other chips depending on quantity. Separating any of those into removable boards will necessarily make phones larger, heavier, and otherwise suboptimal.
> You should also have a standard battery connector
Modern devices often don't use a single discrete battery; they distribute battery cells through a device anywhere they fit and wire them together.