Oh, I know that glass isn't quite as fragile as most people assume. The problem, however, is that D&D mechanics puts glass Hardness at a 1, while iron and steel is a 10. The Mohs scale doesn't apply at all in this case. Hardness from a D&D perspective isn't about actual hardness, but about resistance to a meaningful amount of damage. Not the best way of doing things, but it's what we have to work with.
If DP buffed glass to have a Hardness of 5, though, that would be great. Combine that with Hardening Blood Wished by Viserys and the resulting product would have a Hardness of 13, which is much stronger than iron or steel, and getting quite close to Mithral's 15. Even if he doesn't, a Hardness of 9 is still quite durable.