My first thing with 8051 was designing a resident monitor-
replacing the need for ICE´s etc.
And well seasonded hands out there in the vineyards of
electronics know why.

A resident debugger ist most useful in combination with a ROM- simulator.
I am a "seasoned hand" (did actually work with HMOS) and your postulate is totally false.

In the olden days NOBODY would use as much as ONE byte of that expensive commodity known as memory to "replace the need for ICE´s"

Today you do not even consider memory use a factor, but in those days it was THE issue.

Erik