Additionally even the build-up costs to ISPs to get to these residences is per-capita small, because multitenant housing means that even if only a minority of units subscribe to the service, the costs to reach the premises is still spread out among multiple customers.

Granted, most of the carrier-integration I worked with was either metro optical ethernet or much older frame-relay connecting to points-of-presence where OC3s interconnected those points-of-presence, but in either case it was set up where the service provider came into a service-entrance room, and cabling left the service provider's equipment for some kind of physical demarcation point that acted as the official split between the service provider's responsibility and the on-premises tenant or property owner's responsibility. In our few multitenant buildings the service provider would do some on-premises work and actually place that demarc in the customer's own suite. This was not generally seen as being all that big a deal to do.

I would expect that large apartment complexes would already have a dedicated room for headend equipment, and that the service provider would use some other medium besides coaxial cable to get from that headend back to the central office, and between that headend equipment and the customer cablemodem would be normal coaxial cable run within the complex. Absolutely it is not a sure-thing that there would be sufficient customers to make this profitable, but that's a business risk that the provider should accept on getting into this line of business. We're not obligated to make their business model work for them, that's their responsibility to figure out.