if it relies on natural gas to provide the energy input at any point.

Solar panel manufacturing using solely energy from solar panels, perhaps?

If everyone invested in solar panels in the State of Maine, I suspect the majority incremental cost for the networks would be largescale battery back-up for evening/overcast weather.

I'll leave the math to you.

But how do you cost replacement generation? What's a new powerplant cost from construction thru fossil fuel costs? Decentralized energy seems to be the right direction to go with.

You would be imagining the great state of Maine.

In 2021, 72% of Maine's total electricity net generation came from renewable energy, and hydroelectric power provided the largest share at 27%.
In 2021, Maine ranked fifth in the nation in the share of total in-state electricity generation provided by wind energy, and wind provided about 23% of Maine's in-state net generation.
Forests cover about nine-tenths of Maine, a larger share than in any other state, and wood and wood waste-derived fuels supply most of the biomass Maine uses for electricity generation. In 2021, biomass supplied one-fifth of Maine’s total net generation, the second-largest share, after Vermont, among the states.
In 2020, Maine’s natural gas use on a per capita basis was third-lowest in the nation, after Hawaii and Vermont, in large part because most of Maine lacks natural gas distribution systems.

https://www.eia.gov/state/?sid=ME

Labor intensive, decentralized solar power makes a lot of sense for the future in Maine. We can be a leader.

On edit: We have a lot of hydropower, too.

source without a renewable energy source. The transition will either happen using present energy sources or not happen.

The benefit calculations need to include the non-renewable energy sources used to create the renewable energy hardware, like solar panels and wind turbines.

They all have a limited lifespan, and until the equation falls in favour of renewables to power that hardware replacement and recycling of scrap, it's a bit of a pipe dream.
But the future of the human race depends on it, so keep going!

Of course if I was a Science Fiction writer, I'd conjure up mini clean nuclear energy power sources, suitable for homes and vehicles.