There's a lot of much lower hanging fruit. As you state: there is no reason the source has to be parsed from scratch every single time. A little memo-ization of intermediate data structures when the underlying source has not changed could go a long way.
AFAIK Microsoft's toolchains do this but they don't do it well. It's a frequent source of headaches.
I think this is an area where practice has lagged quite far behind theory (for entirely reasonable practical reasons). I've seen quite a few papers relating to incremental parsing, compilation, etc. (e.g. [1,2]) but afaik very few build systems actually take advantage.
That can create wildly different includes. The upcoming modules feature in C++ may change that. But unfortunately at this point we get to live with a recompile.
Pre compiled headers took some of the sting out of it. It would be nice if they could say 'in this case at this point these flags existed and I used them I will hold onto that for this header'. That way you compile it once and the rest of the project can change things in and out. The pch way seems more 'i know I will not change the flags ever'. It was a good start. Maybe a new object type and it could even live on between compiles?
dunno why you got downvote, I thought your point was pretty legit. IMAO, we might even need to change common binary file formats (e.g. ELF, COFF) to accommodate this sort of feature
AFAIK Microsoft's toolchains do this but they don't do it well. It's a frequent source of headaches.