> Users still have access to the same exact code. There is no loss.
Sure, there's loss of what the other party builds on top of the code. While I don't think this is a real loss, the argument that it is is exactly the argument that Stallman uses against the desirability of non-copyleft licenses, so Stallman's positions on these issues seem incoherent viewed together, even if each might be viewed as reasonable enough on its own.
Isn't selling the exception explicitly enabling the proprietary extension, rendering the dual licensor essentially a partner in profit regarding the proprietary software?
It doesn't enable proprietary extension any more than permissive licensing does - in fact, less so since it comes at a cost. Meanwhile, it also provides some funding for Free Software development. I can't imagine how that would be worse than a permissive license.
Sure, there's loss of what the other party builds on top of the code. While I don't think this is a real loss, the argument that it is is exactly the argument that Stallman uses against the desirability of non-copyleft licenses, so Stallman's positions on these issues seem incoherent viewed together, even if each might be viewed as reasonable enough on its own.