PON faces the exact same constraint, and because the core of most cable carrier networks is carrier fiber my experience is that new-installation CMTS typically have better transit (multi-gbps fiber) than PON (1gbps fiber common). This is for smaller municipal PON carriers, I know less about major operations like AT&T's and Verizon's.
You can actually run DOCSIS over fiber too. I used to work for a mid-sized regional ISP that had everything from GPON, to DOCSIS cable, to every flavor of DSL. The backhaul throughput is the constraint everywhere, but so is signal loss over distance. The limiting factor for urban areas is usually oversubscribed and aging equipment (and building out new lines is very hard to do in an already developed area because legal/bureaucratic reasons), and the limiting factor for rural areas is the sheer distance needed to cover to get to maybe a couple dozen potential paying customers. The vast majority of the US is empty/undeveloped space with dozens to hundreds of miles in-between.
