C
Chad Perrin
The major difference from the "forever opensource" point of view is
that BSD license does not require you to distribute the code in source
form.
This allows you to only distribute the binaries of a modified version
of the software and keep the source, making the software as
proprietary as it ever gets. People can disassemble it, but they can
do the same with proprietary software (the license may forbid it but
it is unenforcible technically and often even legally). To disallow
copying the binaries just link with a proprietary module.
Software gets a *lot* more proprietary than "only available as a binary".
It's also rather difficult to get existing copies of source out of
circulation, so taking a copy of the publicly available source and
compiling it, then distributing only the binary, doesn't really
accomplish much in terms of making it more "closed", in practice.
Your comment about what is and is not enforceable strikes me as premature
and lacking strong supporting evidence.