$15 probably does cover the costs of these plans. 25 megabit?
I would just assume that if you want to provide broadband service to low-income that the government would be making up for some of the lost revenue.
A low cost landline was $10/month 10+ years ago, and usage was extra. I think that's an ok baseline for a connection that needs maintenance and service.
I just don't think you can do that for $15/month, even if the capex of build out has already been paid for. Maaaybe if you had near 100% uptake, like landlines used to have. But no ISP is getting that. Most (certainly not all) people can choose between wired internet from a telco and a cable company, and 4g/5g is available in a lot of places, starlink in less dense places, and a lot of people don't need a separate home internet plan because the only networked device is their phone.
Given that more than half of the population of NY is in NYC and Long Island, I would imagine that there is enough density and economic diversity at least downstate to make this at minimum cost neutral though this is total conjecture.
As an aside, I remember the days of companies partnering with CLECs taking advantage of rural connectivity loopholes to create free conference calling services by billing larger upstream carriers and splitting that revenue with service providers, which was pretty clever.
If it's cheaper to build out their own fiber and not have these plans, that is an option these companies have, too.
This comes down to an argument about what "cost", it will cover the marginal cost of an additional subscriber sure (e.g additional customer support, sending a router out, taking a payment each month, 500GB/month across the backbone etc) but will not really be enough to payback the capital cost of the original fibre or coax rollout.
It can once you start adding up all the customers and worrying about 'upstream' connectivity.
One thousand customers at 500Mb can potentially saturate a 400Gb link; one thousand at 25Gb cannot. One has to do capacity planning and average and worst case scenarios to worry about, especially at peak times.