What version of DOCSIS are you dealing with? Any what number of customers on a CMTS port? Coax might last longer (citation needed, I don't see coax undersea), but how long to the nodes that are needed every couple hundred feet? The only parts that need electrical power in a PON are the ends.
10GPON easily beats the highest tiers of DOCSIS specs (giving the benefit of the doubt by using the theoretical numbers) and even that can't provide a gig per customer once it gets reasonably provisioned with customers.
The biggest problem with fiber's scalability is that the light can only be split so many times before it can't be accurately decoded on the downstream (upstream isn't as big of an issue as it isn't being split). On a cable network you just keep adding nodes to boost the signal and a single CMTS port can now serve more customers (but splitting the bandwidth available even more).
From a customer perspective that difference means that fiber will always outperform coax as network load increases as it can't be overprovisioned to such an extent, the highest split ratio I've seen is 1:128 which means split evenly everyone gets just under 20Mbit for normal GPON assuming you can get everyone proper signal at such an extreme split ratio.