The trouble with subsidies is, what happens if there is nothing to subsidize? In sufficiently rural areas there may be zero existing providers, and still be zero providers even if you offer a modest subsidy. But if you offer a large subsidy then the costs quickly get out of control as less rural areas that needed some but not that much end up claiming more subsidy than was actually necessary, and regulatory capture encourages the government to shovel more public money to private providers.

You also have trouble with suburban areas that can justify fiber but not twice. If there is only going to be one provider with no competition, better the municipality than Comcast.