Lots of very interesting points being made here and I can certainly see the value of the touching experience but at what point do museums decide what is acceptable to touch & what isnt ?
If for example you have a modern(ish) type that you put on display straight off of squadron service without any form of restoration and you allow people to touch the airframe, over the years you could potentially end up with the surface being eroded “a la Flak bait”.
As BSG75 mentioned it, i’ll use the SR71 at Duxford as an example of this. This is an “unrestored” airframe (as far as I know) but can be touched.