To "simply run out of fossil fuels" is like that potentiometer you mention, it isn't like you run out all at once but you run out of the cheap ones first and it gets more expensive.

I remember reading

https://www.amazon.com/Hubberts-Peak-Impending-Shortage-Revi...

in the early 2000s which was about the coming peak of conventional oil production and it turned out to be wrong in the sense that we knew in the 1970s that there were huge amounts of oil and gas in tight formations that we didn't know how to exploit. People were trying to figure out how to do that economically and had their breakthrough around the time that book came out so now you drive around some parts of Pennsylvania and boy do you see a lot of natural gas infrastructure.

I remember being in my hippie phase in the late 1990s and having a conversation with a roughneck on the Ithaca Commons who was telling me that the oil industry had a lot of technology that was going to lift the supply constraints that I was concerned about... he didn't tell me all the details but looking back now I'm pretty sure he knew about developments in hydrofracking and might even have been personally involved with them.

We have still lot of known fossil fuel reserves. More than we should put into atmosfere in form of CO2.

Coal for 139 years

Oil for 56 years

Gas for 49 years

https://ourworldindata.org/grapher/years-of-fossil-fuel-rese...

This is a bit simplified because high fossil fuel prices also drive inovations in mining, exploration and could increase known reserves.