> But this is not a matter of selling proprietary licenses to fund open source code, which strikes me either as a propagandistic or else a quaint way of seeing it. Rather, it's the problem of abusing copyleft specifically to sell proprietary licenses, with a number of specific attendant problems, such as promoting FUD around copyleft.
The problem is how to you block the bad behaviour (which as you note is not especially popular to start with) without blocking the acceptable versions.
your 'Anti-MySQL' clause isn't any better than the FSF's 'Anti-Tivo' clause. both are reactions to fairly rare abuse cases, and both are divisive in that there are large numbers of people in the community who don't see the behaviour as being so wrong that it's worth complicating a licence (and loosing all the non-abusive variations of the behaviour) to block the abusive behaviour.
> A business (or its legal advisor) contemplating some copyleft-based open core scheme or the like will take one look at copyleft-next and decide to use some other copyleft license for such a purpose.
that assumes that that approach is in their minds at the start of the project. It may be something that's first discussed years into the project (when it gets big enough and functional enough for people to be interested in paying for it)
It's very common for projects to start and grow, and then hit a crisis where the work to maintain them is large enough to need the core people paid to do the work, but not have a funding source in place to do this. It's times like this that the 'run it through the GPL to strip this clause' would happen.