I'd also suggest that the exit will be a long time coming, perhaps not even occurring by that 15-20 year mark and that the effect of staying atop AQ recruiting posters is of little to no concern.
Which is one of many reasons why the so-called War on Terror is doomed to last forever. We don't address the underlying causes for terrorism to flourish.

It's also a good example of why it took so long to figure out a COIN strategy that worked; a strategy that should have seemed obvious at the outset. We're making the same mistake in the GWOT. Where's our COIN strategy for that "war"?