Fossil fuels are seed capital, yes. Modern civilisation has quite the burn rate.

I like to consider the technological stack height for various energy systems. For nuclear, as you note, it's quite high. One thing this creates is systemic risk of sheer system failure and inability to reboot. One of the consequences of the Fukushima disaster and following shutdown of much of Japan's nuclear power sector was that the entire nation faced a shortfall of electrical capacity. Not helped, incidentally, by its dual incompatible power grids, one operating at 60hz, the other at 50hz, limiting inter-tie capabilities.

While fossil fuels have a fairly low tech stack requirement, it's not zero. For a long time, plant-based sources of equivalents: olive oil and fuelwood, principally, were preferable. They could be obtained locally, processed readily, and renewed themselves.

For local use, all you need is a coal face or oil seep. Gas, without industrial means to capture, transport, and contain it is quite problematic, though the Chinese actually did just this, nearly 2,000 years ago, using bamboo pipes and hide bladders[1]. I hate to ask what the fire incidence was....

Oil didn't take off until an existing coal-based industry provided iron, and later, steel, for drilling, storage, pipes, railroads, and ships. This industry developed incrementally from roughly 1860 to 1900, though even _quite_ early in the process, oil equipment took on much of its modern appearance.

The other thing oil wanted for was a practical application: the internal combustion engine, which required: roads, tires, rubber, steel, electricity....

Another challenge is that fossil fuels hugely distort prices. We price fossil fuels at extraction effort, but the long-term cost is the replacement cost. Since replacing fossil fuels with more fossil fuels isn't viable (we don't have 100-500 million years to wait), that generally means existing biomass. Humans appropriate roughly 40% of that now, and it produces at net efficiencies of 1-3% of incident sunlight converted to fuel (algae might boost that to 10%), which has highly specific land, nutrient, (generally fresh) water, and climate requirements.

Nuclear seems to offer a possible out, but for any fission-based process is also* a nonrenewable option. Estimates of fuel abundance range from 80 years to a few thousand, at present rates of consumption. Relying on uranium for all present human energy needs would exhaust reserves in 6 years. Breeders offer a 100x improvement in fuel utilisation, but that pushes us from 6 years to 600 (without growth), or 80 to 8,000 at present rates -- a goodly interval, but only slightly longer than our current history. Suggestions of recovery of uranium from seawater might extend lifetimes further, but with another expansion of the the tech stack requirement. And there's the near-term immediacy of the problem to boot.

I'm leaning to the side that says that reducing our overall throughput (smaller population, decrreased per-capita affluence, possibly both) is going to have to be part of the solution. That's going to be unpopular. Possibly thermodynamically improbable without massive systemic changes, possibly catastrophic.

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Notes:

1. http://www.computersmiths.com/chineseinvention/natgas.htm