Do you know what other prior systems did for co-processor instructions? The 8086 and 8087 must have been designed together for this approach to work, so presumably there is a reason they didn't choose what other systems did.
It is notable that ARM designed explicit co-processor instructions, allowing for 16 co-processors. They must have taken the 8086/8087 approach into account when doing that.
It is notable that ARM designed explicit co-processor instructions, allowing for 16 co-processors. They must have taken the 8086/8087 approach into account when doing that.