when Colin Campbell was writing of peak oil and Simmons was inferring that US NG was peaking, not much was written by either of the two that hasn't been confirmed in history.
Campbell stated that sweet crude would be peaking soon and that we wouldn't know exactly when until after it happened. He spoke of a period of plateau when other less easy and economical forms of oil would fill the gap for a time, but in the end the decline would continue. This would cause a substantial hardship on developed economies that would progressively get worse as newer forms of energy probably would not be able to make up the demand difference. I think this has pretty much been borne out in what is happening since 2008, with the development of tight oil to temporarily fill the gap extending the plateau as global coal use has replaced some of the past demand as well.
As for peaking NG, conventional NG appears to have peaked in the US near the time Simmons was warning, and only through the controversial practice of fracking at the cost of environment, has the peak been pushed back several decades. But, it comes at the cost of environment, property rights, and the deregulation of the gas industry through blind eye EPA enforcements.
Personally I fail to see where peak oil or nat gas has been disproven. I think it is happening now in the plateau, but the cliff is still out there in our future, and coming faster than replacing the expanding energy demand can be realized. Not much I see happening today, changes what I believed back in 2000, except the added challenge of faster than thought probable, change in the climate, adding to the potential for hardship for economy and society.