The all-in efficiency of plant-based photosynthesis -- about 1-3% for most land plants, possibly as high as 10% for algae.[1] The all-in efficiency of solar energy is close to the same magnitude -- panel efficiencies are as high as 37% (single-layer), in practice closer to 15-25%, and with spacing, capacity, and other factors, tend to fall to about 5%. To which manufacturing, installation, and disposal costs (all self-provisioned by plants) are required.

The point remains that for primary energy, we don't have a whole lot of options, and plant productivity is much of that. A modern civilisation knocked back won't have nuclear power, reliable electrical transmission, or advanced battery technologies. Mobility of fuels will be limited.[2]

"Technology", a distractingly vague term, seems to be based on specific modalities, each with its own specific capabilities and limitations.[3] Precision boring is contingent on sufficent-quality metalurgy, based on available materials, fuels, and smelting/casting controls, as well as measurement and feedback (information/systems management) controls. At that, what you buy in, say, a reciprocating steam engine, is a net fuel-to-work efficiency gain from about 3% (Watt ~1800) to about 25% (best stationary triple-expansion engines). Locomotive steam engines were about 10% efficienty, barely 3x better than Watt's 1800 design.[4] That is: precision and technology still only bring you up to theoretical maximum efficiencies. Process and systems management technology is at best a fractional factor 0 <= t < 1. (Many proposed technologies end up with efficiencies well below 0. We typically don't adopt those.)

Fuels are larger multipliers. Increasing applied energy frequently increases potential output as a multiple factor: capital + process + n times more energy results in some tn times more output (where n > 1 and 0 <= t < 1). You can keep applying more energy to a process for ever increasing outputs, at least to a considerable extent -- generally to thermal, friction, vibrational, stress, or other limits.

Landfill mining has been suggested. It was a (minor, though not miner) element in Arthur C. Clarke's Imperial Earth* (1976). And has long been practiced, historically and presently, with a range of consequences.

The more general problem here is that recycling is at best less than 100% efficient, and the generational loss is r^g (g == generations, with 0 <= r < 1). The most-recycled current materials within the US is lead, largely from car batteries), with rates of 74%. At two generations, the remaining material is 55%. After 10, it's less than 5%.[5] Eventually dust is your best source. Non-ore-forming materials (e.g., rare earths) already exhibit this, the difficulty in their sourcing isn't due to their scarcity per se (they're relatively abundant within Earth's crust), but in their failure to concentrate, and the immense amounts of overburden and tailings created in their use. It's less a matter of mining than of pollution and landscape spoilage exporting, costs China have been willing to absorb in recent decades.

The next H-B alternative, not dependent on abundant natural gas,[7] or lack of one, will be hugely monumental. I'd hedge my bets on a bioengineered process, though there are others being pursued, see:
https://www.nature.com/articles/srep01145

https://www.sciencemag.org/news/2018/07/ammonia-renewable-fu...

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Notes:

1. Both based on incident sunlight => stored energy, generally as cellulose or lipids. Algael productivity discounts the likely impact of parasites and/or disease on large-scale development (as does land cropping). Lifeforms other than human are also keen on large and convenient plant-energy stores.

2. There's little discussion of this even in histories which take an energy-centric viewpoint such as Smil or Weissenbacher, but a key liability for fossil fuel energy resources was that they were so unevenly distributed, and transportation capabilities so limited. Better to (locally) gather fuelwood, dung, press olive for oils, or render animal tallow, than to try hauling seacoal, natural tar, or the few rare oil seeps more than a few tens of km.

Even the coal-rich US didn't transition from wood until its inland rail-based transportation system was developed. On which coal still represents a large share of revenues and massive constituent of total ton-miles. Oil's development as a fuel waited in part on reliable metalurgy, the capability to construct pipes of more than ~10cm in diameter and a few tens of metres in length. Along with rights-of-way, a sufficient challenge that as of WWII, major infrastructure (the Big Inch and Little Big Inch pipelines, still in use) were constructed not by private industry but by the Federal Government.

International oil shipments were contingent on developing double-hulled supermassive oil tankers. Ari Onassis, Greek shipping magnate, was, it turns out, a Greek supertanker shipping magnate.

After water, sand, and gravel, oil is the most-transported material in the modern economy.

Natural gas likewise is more difficult to transport than oil or coal, requiring hermetically-sealed containment, and high pressures and/or low temperatures. It has a tendency to go boom in ways that coal and oil for the most part don't, which makes neighbours even edgier than for those materials.

Transport and nonuniformly-distributed fuels are co-dependent.

3. My thinking leads to identification of nine of these: 1. fuels, 2. materials, 3. process (technical) knowlege, 4. causal (scientific) knowledge, 5. network dyanmics, 6. systems management (physical, social, informational), 7. information (acquisition, parsing, processing, storage/retrieval, transmission), 8. power transmission & transformation, and 9. hygiene (addressing unintended / undesired consequences). Still in development. See (in slightly earlier form): https://ello.co/dredmorbius/post/klsjjjzzl9plqxz-ms8nww

4. Vaclav Smil, Energy and Civilization, (2017). Figure 5.5, p.243.

5. https://www.usgs.gov/centers/nmic/recycling-statistics-and-i...

6. A chief export of Trinidad and Tobago, and the reason for that small Caribbean island nation's hugely outsized per-capita carbon footprint.

See:

T&T EMissions Trends:
https://cdiac.ess-dive.lbl.gov/trends/emis/t_t.html

T&T CO2 emissions
https://www.worldometers.info/co2-emissions/trinidad-and-tob...

Global per-capita emissions
https://2oqz471sa19h3vbwa53m33yj-wpengine.netdna-ssl.com/wp-... (from: https://www.visualcapitalist.com/all-the-worlds-carbon-emiss...)

T&T exports / economy:
https://www.worldatlas.com/articles/what-are-the-major-natur...