Its silly to believe it would go on forever. Nothing does (laws of thermodynamics and all that). The bigger question is will it last “long enough”? Based on the information we currently have the answer is a “probably yes”.
Yes, we need oil/etc for many many other products other than just as a way to produce energy cheaply, but even when it becomes so scarce that its not an affordable energy source we will still have plenty enough of it to produce all of those things. It’s the energy use that is the mega “user”. It’s already technically possible to replace gasoline as our source of fuel for transportation (it's just not very affordable). We already currently have the technology to generate more than enough power to supply the country, including enough power if we all switched to electric cars. We could do it with Nuclear, and we could do it with a closed nuclear cycle that produces almost no waste and recycles the vast majority of it back into fuel (so that we wouldn’t need to mine and enrich uranium).
The big economic issues would arrive if this all happened suddenly and without warning. That doesn’t seem very likely. What’s far more likely is technology will continue to improve in all of these areas and as oil or NG becomes more difficult to extract (and thus expensive) alternatives will continue to drop in price and increase in efficiency (like we are seeing with solar and batteries). Once they meet and oil/NG starts to become more expensive than solar/wind/nuclear/etc.
Do we need to use less oil/NG? Of course we do, and we will (one way or another). The market will see to that. But will it happen in some terrible USA destroying apocalypse? No, not likely.
I also think its silly how many people hypothesize these scenarios but single out the USA like the country would fail and not somehow take the rest of the planet along for the ride. The unfortunate reality is if something like that *did* suddenly occur it would be quite bad for most all of the world (suddenly losing nearly a quarter of the planets GDP tends to do things like that).
