The main problem with trying to revert sites to 'how they were' is knowing how far back to go. I think it was Josh Pollard, when asked if Avebury should be restored, said "When to? The Mesolithic?" or something similar.
Monuments are as monuments are. A great many need tidying up and maintaining. A few could benefit from clearing trees etc - Wayand's Smithy to name the first big UK example that springs to mind.
The last thing we need, though, are more Newgranges. All I'm saying is we don't have to make anything worse. Countering that with - "what about all the trees that are there already and shouldn't be", is pointless. The issues are totally separate. One is done. The other can be stopped.