Your post actually highlights the problem. What makes 10% the proper profit margin? BT OpenReach, the U.K.'s regulated infrastructure monopoly, has a profit margin of double that. That number becomes a political football, and the political result probably isn't what most people on HN would want. People are happy with 25 mbps DSL; they're not going to vote to raise Internet rates to drive returns high enough to incentivize investment in replacing everything with fiber. That's exactly what you see in other rate-regulated utilities. People don't vote to replace lead pipes that poison kids, because they would rather have cheaper water rates; they don't vote to replace sewers that leak raw sewage into rivers when it rains, because they'd rather have cheaper sewage fees.
I'm not sure how not having cheap fibre to ge doorstep is an externality.