The post-peak decline rate is another question. The best guides we have are the performances of oil fields and countries that are known to be already in decline.
The problem with many forecasts is they attempt to extrapolate decline of the entire world from the rate of individual fields. Long after peak, new production will still come online from existing reserves, new fields and new efficiencies. Extrapolating from limited examples only shows the down side and doesn't take into account any upside. Alaska and fracking are both bumps just as russia had bumps and Mexico and Venezuela both have had declines in part due to bad management and little investment.
the North Sea basin is showing an annual decline around 10%, and the giant Cantarell field in Mexico is losing production at rates approaching 20% per year
So what? Iraq is increasing all the time, Tupi (Lula now I guess) has hardly been tapped and one day the arctic may be a placid as the pacific... not to mention there will be lots of small fields remaining to be found and maybe some big ones - though big easy ones will probably be scarce.
The other thing is that geology may hold the final number but everything else is in play, from politics to economics to social status. 10 years ago no true peaker would have predicted that driving in the US would be down 8%, the recieved wisdom was that we'd drive to survive (
good song BTW) or die trying. Perhaps historically demand is only impacted x% at $x per barrel increase, but there is no rule that says it will only be affected 2x% if the price doubles, perhaps a doubling of price will reduce demand 4x% or 8x%.
There is no way to guess what's in front simply by looking back.
That is the hard part about forecasts, imagining exactly how much price will affect demand, price can not increase infinitely, so consumption in the face of increasing scarcity can not remain constant. If anything that is the biggest hole in the whole peak oil forecasting ...framework(?)
But I assume that wasn't your point in posting this was it "cottager"? Seems more of strawman construction to me, i.e., it hasn't declined, hence it never will.
The legitimate object of government, is to do for a community of people, whatever they need to have done, but can not do, at all, or can not, so well do, for themselves -- in their separate, and individual capacities.
-- Abraham Lincoln, Fragment on Government (July 1, 1854)