> How do flame graphs handle the case where most of the time is spent in some leaf function that is called from all over the program?

In my experience, they don't really. They're very good for finding easy wins where a high/medium-level operation takes much longer than you'd expect it to, not so much for finding low-level hotspots in the code.

> such a view would [...] subsume the usual use cases for a flame graph, would it not?

I don't think it does. A flame graph gives you a good idea of the hierarchical structure (and quickly). Sometimes you can work that out from a DAG, but not necessarily:

Let's say that A and B call M, and M calls X and Y. All four edges have roughly the same value. A flame graph can show you that 90% of the time spent in X started with A and that 90% of the time spent in Y started with B, but the DAG can't.