"If it was torn down and a proper memorial built such as the WW1 Menin Gate Memorial (Ypres);"
There are plenty of well-preserved sections of trenches that you can go and see in France. In fact I think there is one at Ypres.
"Perhaps we should wheel out the block and axe onto Tower green, or the gallows at Tyburn, or perhaps the rack even, plenty of tourist attraction in these. "
You can see all kinds of stuff like that at the Tower of London.
I've visited Belsen and Sachsenhausen. DVDs and books don't hammer home the reality of it in quite the same way. The fact is that while man hasn't necessarily learned from his own inhumanity, most of it isn't that well preserved - only documented. There's not much left apart from records and some archaeology to testify, say, to Ghengis Khan's atrocities. In the case of Auschwitz, we have something that's very rare - a more-or-less intact remainder. I
Plus the camps I've been to certainly aren't just grisly tourist attractions as you seem to think they are - Belsen, for instance, is like one big memorial. Sachsenhausen's different but it's still very strongly geared towards reflection. While humanity can't wear a hair shirt over its atrocities, it's not often we have quite so tangibly preserved an example of what people are capable of. If we got rid of it, I think it'd be a long time before we had anything like it left.