When the temporary becomes permanent

In his essay, “In Praise of Impermanence” (Wall Street Journal, April 20, 2012), Eric Felten makes a plea for the construction of temporary buildings, noting that buildings which are not expected to last for the ages are more likely to be experimental and ultimately architecturally influential. He argues that the Eiffel Tower which was planned as a temporary structure but quickly became an integral part of Paris would probably not have been approved if it had been proposed as a permanent structure. Innovation is wonderful, but what will be the conservation consequences if many other temporary structures end up as permanent ones?