Dunno, maybe you're right and it would have been safe to leave Seahenge in situ (for a while at least).

We keep coming back to the same issue though. If the Elgin Marbles and the paintings and manuscripts from Tun-huang had been left in situ they would almost certainly not be in the excellent state of preservation they're in today (nor so accessible to the world). If Sutton Hoo had not been excavated the world would never have seen some of the treasures of early Anglo-Saxon England. And if Seahenge were washed away and Abu Simbel forever submerged who's to say that future generations would appreciate our reluctance to save those things.

Not sure if there's an answer to this - guess it depends on what's important to your belief system.