I don't object to the principle you've suggested here, though I also don't see anything wrong with projects that choose to allow that practice. (I see nothing wrong with the "share your code or pay us" licensing model, and I certainly wouldn't consider it an error in all previous copyleft licenses.) However, I'd much rather have seen that as an optional addition to a license, rather than a required part of an otherwise much simpler license; as long as this provision remains in copyleft-next, I'll stick with the GPL. Given that you started this project partly in response to the many novel and controversial licensing provisions in GPLv3, such as the anti-DRM clause, it seems suboptimal to introduce your own novel and controversial licensing provisions.
The main reason I don't consider this an error in previous copyleft licenses, and the reason I brought up assignments/CLAs: the copyleft licenses themselves have no inherent asymmetry. If you write a project yourself, and hold all the copyrights, I see absolutely nothing wrong with choosing a dual-licensing model, including one that provides paid non-copyleft licenses. If you accept contributions, then unless those contributors sign an assignment or CLA, you can no longer use that dual-licensing model without the agreement of those contributors. If you want to maintain that dual-licensing model while taking third-party contributions, you have to introduce an assignment or CLA, and *that* introduces the asymmetry; the copyleft license doesn't.
I see no asymmetry here, and thus I see no bug in existing copyleft licenses here.