If society ever crumbles, it might never reach the same level
Think about it - all the most easily extracted fossil fuels, the stuff near the surface, are already exhausted. And, to transition to sustainable energy, you have to bootstrap manufacturing with fossil fuels, since mainstream sustainable energy requires photovoltaic cells and controllers and electronics.
Also our technology is built on layers and layers of incredibly specialized knowledge, much of which is held by a relatively small number of people and only stored digitally.
Also pretty much any manmade object that you use is made with very precise tools and of highly processed materials. Even if you had someone with all the knowledge to make the thing from scratch, and all the raw materials to make it and make the tools used to make it, it could take decades to create from scratch.
Yes. We live in the Anamoly. Never before has humankind had so much energy at their hands and likely it will never again. Unless we make the transition for renewable energy. It's looking a bit more positiv in that regard, but we will see if it's enoth to avoid a collapse or simplification event.
This is related to the diminishing energy returns on energy invested (see ERoEI). It’s a measure of how much energy it takes (say 1 barrel of oil) to extract a certain amount of energy (also say 1 barrel of oil). When the first barrels of oil were extracted long ago the ERoEI was somewhere around 100, meaning you only needed to use one barrel of oil to extract 100 barrels. Last I heard, it was down to 10. If that reaches 1 then it’s basically game over for oil. There is still lots of oil far underground/undersea but it costs too much to extract.
We have fossil-fuel free substitutes for a lot of chemical needs. It's just that right now, many of them are more expensive or smaller scale than just riding the petroleum dragon.
If the easy option isn't there, we can still move forward, but it might be slower or require different tradeoffs. For example, plastic might be precious and used judiciously. It might be 500 years from "first hydroelectric dam" to "ubiquitous rooftop solar" instead of 150.