Some of that is one time cost, but you've got ongoing costs like pole maintenance and replacement, repairing breaks, trouble tickets, call centers, etc.

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.