[robbrown451]

I think most people understand that the stone vaults are what is seen from the inside. At least most people who have given it a tiny bit of thought or looked at any pictures. Most, but not all, of the stone vaulting remains intact, although some of it may be weakened.

While they may comprise the bulk of the structure, I don't think it is correct to say suggest that the roof and roof supports (all that timber) is merely to keep the rain off. It plays a significant role in the structure as well. Sure, without an earthquake or strong wind the valuting is strong enough, but with the roof supports gone the building is substantially less able to stand up to extreme conditions.

As for replacing the roof with something else, it would have to be done where it contributes to the structure, not just sits on top. Glass can actually be structural, in fact. I like some of the new ideas, and don't think the ones I've seen are unfeasible, but they would simply (or not so simply) be done with a whole lot of regard for adding to the structure.

[blahedo]

I think your middle paragraph is much more true for modern (last 100-150 years) buildings than older ones, though I'm not an architectural engineer.

When you say "glass can actually be structural", what meaning of "structural" are you using?