The stackful coroutine route is ideal on paper, but in practice it tends to perform poorly if you retrofit existing code -- because quite often, that existing code depends on blocking i/o or relatively long computations; e.g. opening a file, reading data from a database, or waiting for a TCP connection on the other side of the world to yield data.

It is, of course, possible to wrap all of them in a stackful coroutine interface - but that is no longer "retrofitting existing code without change". My impression is that the cumulative experience is to go either with event driven (in C / C++ / Python) or Stackless (in Python / JavaScript) or threaded (in Java), but stackful coroutines very rarely deliver on their promise.