Erlang essentially using that hybrid model. The runtime uses a shared heap for strings and some other immutable data that can be reference counted. On top of that each thread has own GC-heap and messages copies everything private heap.

I am puzzled why other languages do not even try to explore this. This will be a nice fit for Ocaml or nodejs. And even with Go I wish for be an option to run several Go processes with a shared heap with explicit API to write/read things there and channels working across processes.