Glass Darius is bad. If the enemy team can't get their act together long enough to focus down Glass Darius before he can maximize his ult, then they're going to get what they deserve.
Who said anything about Glass? Van Dams increases survivability as well as damage. Can grab one or two damage items or hybrid damage items on Darius and just beef from there.
My point is simply that Darius can do quite a bit of damage without ult and doesn't need a ton of itemization to do so. He's particularly dangerous, even without any damage itemization, toward mid game damage which is what TT revolves around anyway.
Comparing Darius's damage outside of Ult to a sickly kitten with asthma is extremely hyperbolic. Sure his Ult is super important but that doesn't mean he starts hitting for nothing without it.