My understanding (which could certainly be incorrect) is that most fossil fuels formed between the evolution of trees and fungi that could breakdown lignin and so most fossil fuel deposits were ancient even to the dinosaurs. I doubt that significant stores would form between now and the deadline for life on earth in a few hundred million years.
Yeah, (un)fortunately simple math is all you need to see the carbon cycle could not possibly have worked that way. There isn’t enough carbon produced under the proposed scenarios to support hundreds of years of tree growth, let alone millions. There was probably other stuff that broke down lignin.
>most fossil fuels formed between the evolution of trees and fungi that could breakdown lignin
This is only true for coal, not for oil (which is formed mostly from sea plankton biomass that is mixed into sediment layers).