The 41 comes from 102.5/2.5.  If a higher (real) discount rate was used, then the number would be smaller (eg at 10% it would be 110/10 = 11).  As you say, it's the point at which an annuity paying at the discount rate would exactly balance the income from sustainably managing the forest if the forest's characteristics allowed the sustainable income to be k times the income from consuming the forest this year.  Probably no forest would stand much of a chance at a 10% discount rate, which would require that the sustainable income is only 1/11th of the immediate consumption income.  (Then I suspect equipment capacity constraints become important also - borrowing to invest in new equipment in order to do more clearcutting will be disfavored also and that provides a negative feedback - I have a whole nother post in mind on the issues with capital investment).

I agree that Maxxam's junk bonds were likely a particularly important feature of that particular situation.  However, the graph I posted above does suggest that the cut-rate generally increased in the 1970s and 1980s.  However, this point could certainly stand to have further evidence adduced.  I intend, when I get around to it, to have a look at the international statistics.  We might posit that the cutting will show some tendency to move around to wherever discount rates are highest.

In a healthy economy, I agree with you that "If everyone saw higher rates and wanted to cut, there would be a lumber surplus and prices would go down".  However, in an economy where resource constraints were causing aggregate supply to struggle to meet aggregate demand, thus sparking inflation, there is excess demand for the products of the biosphere and the high interest rates (plus a high risk premium due to the uncertainties as you note) will provide financial justification for unsustainable consumption of natural capital.  We might imagine a world in which an already slightly strained biosphere is now being used also for biofuels to replace depleting petroleum looking that way.