The GPL promptly makes all my hard work in writing the software valueless (certainly in terms of being able to extract an income stream). You're imposing your "software wants to be free" ethic on me, and not respecting my "the labourer is worthy of his hire" ethic.
In my scenario *I* wrote the software, I want to *sell* the software, and I want to make sure that if I fall under a bus my customers won't be left up s**t creek.
Okay, that obviously means I don't buy in to *hard* copyleft, but your attitude is in the "GPL or die" camp. And you're using copyleft to enforce stuff that is outside the realm of copyright, namely you are (in practice) forrbidding copyright licencing agreements. That's exactly the sort of thing that alienates a lot of copyleft-friendly people who your copyleft-next is allegedly aimed at.
Just as the anti-tivoisation clauses in GPL3 alienated a lot people who saw them as stepping outside the realm of copyright and into the realm of enforcing idealism, so this clause to me smacks of exactly the same.
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. And it's a *contract* matter, not a *copyright* matter.
Equally to the point, I (should) know exactly what I as the copyright holder am letting myself in for. 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. And secondly, it went rather tits-up because they didn't actually understand what they were getting themselves into. But even given that mess, they have a community. If you want to investigate, google "OpenQM". And people, if you find stuff you don't like, PLEASE don't dive in - they are nice people, really!