This sounds a bit like a strawman; I think people are discussing the synchronisation costs for parallel tasks whether embarrassing or not, and specifically the impact of the STW design. And even for embarassingly-parallel tasks, I don’t see people complaining about a reasonable synchronisation tax given the other advantages provided by multicore.
I want to add that the backwards-compatibility argument for the C FFI is a sound one (avoiding hitting the wall of reality). Though it’s unclear how much of the backwards-compatibility claim for the STW design will survive the C memory model issues.