It's not a question of what use-cases theoretically require fossil fuels. The whole problem is the scale.
We have to find a replacement for oil and get it to the scale of oil in a fraction of the time we had to get where we are now with oil. And getting there with oil was easier, because oil is extremely convenient.
It's a bit like saying "we need to rewrite the Linux kernel with a new language that we are yet to invent, and it has to reach feature-parity in 5 years". Sure, theoretically we know how to create a new language and how to write a kernel, but can we do both in 5 years? Ever heard of e.g. Fuchsia? And they didn't try to invent a language for it.
The Germans were able to transition from oil to synthetic fuels while in the middle of WWII. South Africa used the same to provide their energy for decades when the world prevented them from getting oil. We know from those experiences that synthetic fuels scale.
We also know from experience that synthetic fuels are around 5 times more expensive than oil, and so only niches are willing to pay for it if oil is an alternative.
We have to find a replacement for oil and get it to the scale of oil in a fraction of the time we had to get where we are now with oil. And getting there with oil was easier, because oil is extremely convenient.
It's a bit like saying "we need to rewrite the Linux kernel with a new language that we are yet to invent, and it has to reach feature-parity in 5 years". Sure, theoretically we know how to create a new language and how to write a kernel, but can we do both in 5 years? Ever heard of e.g. Fuchsia? And they didn't try to invent a language for it.