But what if the Arduino Uno was released in 1980 ... No USB-UART (which also means no USB programming), a different pin layout (Arduino shields likely won’t fit) and more I/Os than the ATmegas ...

These are aliases of the underlying types, so on Arduino Uno, int8_t is the same type as a char, and uint16 is the same size as an unsigned int. One note, if you’re editing a ‘.cpp’ file ...