The concern is not that the old software magically loses its freedom, but that tomorrow's software will, as the software we develop simply gets adopted as the b

The concern is not that the old software magically loses its freedom, but that tomorrow's software will, as the software we develop simply gets adopted as the base for proprietary solutions that then strangle their still-"free" parents of that valuable development oxygen. In that regard I think even GPL advocates would be quite happy to use your example about Darwin, at least as a cautionary tale (though perhaps there was more benefit for the BSD upstreams than I am aware of; we're still waiting for an open release of ARM Darwin...). Likewise Apple's adoption of KJS/KHTML didn't go as well for KDE as we had hoped.