Regarding Greensboro, Van Halen kicked off their last two tours there, so I guess it can't be
that forgotten.

For North Carolina, I'd probably say some place like Greenville, Goldsboro, or Wilson is more forgotten. Especially Greenville having to compete with the South Carolina city of the same name.
With Springfield, MO, one of my college roommates was from there, so I heard about it all the time. However, it's true it is overshined by STL and KC, and particularly nearby Branson, which only has a couple thousand people but is well-known as a tourist mecca. I think Springfield is certainly more well-known than Joplin, Cape Girardeau, and St. Joseph though.
As for the Rust Belt cities, most of those places are talked about regularly, if for no other reason than because they're so world-famous for being hit hard by tough economic times. Heck, Billy Joel wrote a song about "Allentown". And everyone knows about Flint and Detroit. So while they are losing population, they are certainly not forgotten.