I need to ponder this a bit more but a couple of issues suggest themselves immediately;
1) phaseout of one energy source requires a set-aside for the next energy source. This could mean the descent curve is steeper than the ascent eg using more of the remaining coal to smelt metals like silicon.
2) humans plan ahead for looming shortages.
Wishful thinking.

I think one can be model as discovery of the new resource well ahead of its utilization. I.e for oil we fairly rapidly discovered. For coal it could be viewed as discovering new mines once coal became important. So your set-aside is discovery initially outpacing exploitation of the resource.