Most of the Internet is built this way already. The Internet backbone is mostly idle and under-utilized. About 80% of the fiber that was installed for the backbone has gone unused as technology keeps pushing data transfers faster and faster.
Let's take round numbers - 100 fiber pairs between a pair of major cities, with 80 of them unused, and (say) 1Tbit/s on the other 20 pairs. That's 20Tbit/s of backbone capacity, and you might think of it as 80Tbit/s "unused". However, to bring those fibers into use, you need to sink the capital costs for the routers, optics for the 10 or 100Gbit/s ports, and the DWDM equipment. That's not a trivial cost, and people will need a business case for turning up new capacity.
It's a lot easier to upgrade the core than the edge, but the core router ports certainly aren't sitting there at some low utilization all the time.