wrote
Only by coincidence and only in a 50 ohm system elsewhere
S9 is 50 uV (microVolts) equals 50pW.

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Allison, I'm afraid you are not quite correct about coincidences.

The beauty of calibrating an S-meter in watts is that the result is
independent of receiver impedance.

Of course, there must be a conjugate match between antenna and
receiver. And there usually is. This is taken care of in the
calibration process.

The received signal STRENGTH is indicated in watts, which is all one
wants to know. When reading the meter, who cares about what impedance
the measuring instrument happens to be?

To sum up : There is a transmitter of given power output. There is a
radio path which is an attenuator, And there is a received signal
signal strength meter which indicates watts.

The overall loss between transmitter and power meter may be deduced in
terms of decibels. Professional radio engineers do it all the time.

It can be misleading to think in terms of S9 = 50 uV when one doesn't
know what the receiver input impedance is.
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Reg, G4FGQ