The reason the time is so long is that lots of the drives on our network are spare drives or leftover drives that people are contributing to the network. These drives are essentially free, because they are just lying around doing nothing.
As the network grows, it will eventually outpace the amount of storage that can be provided by idle drives, and the price should go up to something more manageable. ROI of 18-30 months seems pretty reasonable to me.
$30/TB just covers the raw cost of the drive. Add the cost of land, electricity, server racks, etc, and the TCO of a drive rises to probably $60 for 3 years (which is about how long you would expect the drive to last before failing). So the high end of the price is likely to stay below $2 / TB for the lifetime of Sia.