And that is a separate issue.

We are basically just discussing super steel. The quantity may be limited, but once you build a ship with it the ship doesn't need any more.

What we were discussing is power generation. The Federation uses antimatter for that and apparently society is advanced enough to harness and black holes as a power source. Relatively, the Dyson sphere to them is simply to costly to use. Which is probably why Riker had never heard of it.