My personal theory is that the world is about money. And money only. This makes it logical to assume:
- If everybody has bought a combustion engine and this technology has evolved to something almost indestructible (in everyday use), you need something new to sell. (the fuel will be burnt anyway by industry, air/sea transportation, no need for cars)
- To sell it, you need to market it. Politics/influencers are not enough.
- F1 is pure marketing. Making it look as if it would be "sports" is part of the business. In this sense the product is defined by its packaging.
- Consequently, the combustion engine will die out in F1. I assume it won't merge with FE to keep the second already established marketing branch. The two classes will have very distinct sets of regulation.
When? My guess would be 15 years or longer. Things don't change so fast as we are sometimes expected to think. Think of the autonomous vehicle: Almost everybody thinks they are completely evolved for everyday use. If that would be true, they could be sold. How to market them? Present them e.g. as Pace Cars in motorsport. The simplest task one could think of for a "machine" like that. No traffic lights, no pedestrians, the track is abnormally wide, no snowstorms and nothing in the way. But still: No autonomous Pace Cars. Either they are not up to this simple task at all or the producing part of the market is not ready to release them (which can only have economical reasons).