[I think it would be very difficult to make an 8051 API that was flexible enough and universal enough (i.e., "applicable to enough 8051 variants") to be useful, but I don't think the concept itself is flawed. I mean, suppose somebody actually did it, and it was really possible to write something like the following and just know that it would work:

#define VARIANT SILABS_C8051F120
#include "8051api.h"

void main() {
    InitializeUART0(BAUD_9600);
    StringToUART0("Hello, Attila");
}

Wouldn't that be a liberating change from the endless farting around with SFRs that we do now?