Ethical consequences ...

I believe you may be confusing "free as in freedom" with "free as in
beer". After 25 or so years it is settled that businesses can extract
income streams from GPL-licensed software.

> Okay, that obviously means I don't buy in to *hard* copyleft, but
> your attitude is in the "GPL or die" camp.

No.

> And you're using copyleft
> to enforce stuff that is outside the realm of copyright, namely you
> are (in practice) forrbidding copyright licencing agreements.

No, I'm essentially saying 'if you use a strong copyleft license for
some software you write, then if you also want to use a proprietary
license for derivative works of that software, give everyone else the
right to similarly use a proprietary license for their derivative
works". It's an equality principle. Nothing is forbidden; quite the
opposite. You are saying that it is okay for a company to have the
sole right to do proprietary licensing of a GPL codebase -- that is
obviously forbidding GPL licensees from entering into proprietary
copyright licensing agreements covering *their* derivative works.

> At the end of the day, if I am sole copyright holder (or have a
> licence from contributors that says I can) and distribute both
> proprietary and GPL versions, then those other contributors know up
> front what they're letting themselves in for.

It's not the contributors I'm concerned about here, primarily, though
one problem with the model you describe is it tends to discourage
community contributions.

> Actually, I have seen at first hand what happens when a company
> tries to convert their product into a dual-licence proprietary/GPL
> state. Firstly, they would never have touched your licence in a
> month of sundays.

That's exactly the effect I wish to achieve.