Cable companies can't replace network hardware every three years. It's just not profitable. Cable systems were upgraded to broadband 2000-2005. 10-15 year replacement is short in the telecom industry. It's not surprising that new entrants into the market can give much higher speed. If you demand upgrades every 5 years that is going to increase costs considerably.

Also a large amount of coax spectrum is for video tv service.

The real curiousity is why Verizon fios can't deliver near 1gbps. It's already has a fiberoptic network. I'm guessing it's the hardware at the nodes.