Gary Coffman writes:

It exists because of the throttling effect of grid
voltage on plate current. An air regulator has a similar load line,
or lossless dynamic resistance, controlled by the feedback to the
regulator diaphragm and pintle valve.

I know what you mean by dynamic resistance (at 47 today, mine was the last
electrical engineering class in 1975 to study vacuum tube circuit design).
But I don't follow the lossless.

Forget the vacuum tube details like filament power. Assume they are
perfect valves. Electronic charge is conserved in the device, so the
current-in equals current-out. But voltage-out is less than voltage-in
(definition of a regulator or valve). Thus power-out (voltage times
current) is less than power-in, and the difference is the inherent loss.

"Resistance" is force, ergo work, ergo loss. Doesn't matter if it is
"dynamic" or not, or if it is charge or air flowing.