Most of the Dark Fiber left only works at the 100 Mb/s level or lower. Long distance cables are a lot more then a piece of glass, there are repeaters every 100-200 Km, and those are the bottleneck. Since it takes about the same amount of energy to power a 100 Mb link as a 10 Gb link, companies are just laying new fiber that can accommodate 1,000-10,000x the bandwidth with the same operating cost, which is why NSPs are making mountains of cash while their fees keep falling. It works out that if they replace a piece of fiber with 10x the capacity, they can easily charge half as much but make 5x the profit (Assuming 100% usage) and even subtracting the payments on the loan to lay the cable, they still come out on top with a nice profit increase.
As for those old fibers, a lot of them have failed over time, and the rest get leased to companies to run private site-to-site links (mostly telcos for their long-distance back-hauls given the guaranteed bandwidth).