Billions would die and a few more billion would be impoverished if oil magically disappeared. If oil wells ran dry in the medium term (10-30 years?) the world be screwed. Petroleum products are so ingrained in every part of your life, most people have no idea.
You're not wrong, but I think our current level of technology is so basically capable that I think we would actually just adapt pretty quickly, as soon as it becomes .001% economically advantageous to do so.
We would do a combination of both consuming less and developing new forms of what we do consume, especially over a 10-30 year term. I don't mean everyone wakes up anfndecides to care for the planet more than whatever they want to do today, I mean once something becomes expensive enough, we'll just stop doing it.
We've already done things just this year that would have sounded outlandish and impossible. But it eems in fact very little is actually impossible.
The largeness of the numbers doesn't really result in the kind of immovable inertia they imply, for a few different reasons.
1, The huge size of "everything that needs oil" is met by how equally large billions of people are organized into heirarchical structures with magic communication and coordination. Overnight 5 billion people can do something new.
2, All that collossal amount of oil-powered infrastructure is not actually all that long-lived. Ecerything from your toaster to the biggest steel plants to cargo ships to ...everything, it's all actually turning over all the time and nothing ever gets to be 50 years old, just from natural pressure of competition, obsolescense, and simple churn. That means it's really not that remarkable to imagine it all being changed into something else over the course of 30 years, instead of just newer versions of the same things.