DOCSIS 3.1 is capable of 10gbps down, 1gbps up, and the currently less common 4.0 is capable of 10gbps down and 6gbps up. This is equivalent to and often better than a typical FttH PON media (which in the most common form are capable of 1gbps symmetric per string only). The major constraint preventing high upstream speeds is legacy equipment in cable carrier networks; there is progress on changing this but not especially fast. Regulatory action to require high upstream speeds for broadband incentives could significantly speed things up.
The protocol may support it, but remember that the bandwidth shared across and entire neighborhood or market is still only as good as the CMTS is capable of supporting, which is nowhere near that.
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.
The interesting part is that fiber rollout would result in less bandwidth pooling on coax, which would make coax competitive again. I'm projecting it may never entirely go away for this reason- the cables are already buried, and the service will get better as demand decreases.
