Really Shitty Copper

One thing that would have been really tough would be high-temperature pressure vessels, like for heat engines. Other materials with good tensile strength exist (as do wood skyscrapers!), and so do refractory materials, but if you need both at the same time you're looking at either high-tech ceramic composites or metal.

Does silicon even form metallic bonds?

Uhh, not sure. The band structure of the crystal lattice supports conduction like a metal, but not without an impurity to introduce the initial carrier, which is the whole thing behind why it's useful for electronics.

IIRC bond type is kind of a continuum.