"... I can't see the theoretical discount rate applied to these...

"... I can't see the theoretical discount rate applied to these businesses changing much in the future."

Well what do you consider a reasonable discount rate to be? Not so long ago, when some businesses were at a PE of 30, implying an earnings yield of a little over 3%, high grade corporate bonds were yielding over 4%.

Something like an ARB was yielding a dividend of about 1.5%. So where was the margin to low risk alternatives? We can say ARB was giving you 1.5% + the growth rate, sure. But apply any reasonable growth rate you care to, and see how far you have to travel into the future before you get a return that compares well with the low risk alternative (and "comapares well" does not mean matches the low risk alternative).

Try it on a spreadsheet and see how much of a time traveller you have to be. Now if that isn't speculation, what is?