In my personal point of view I have no issue against proprietary dual licensing with GPL.
I would have an issue if the software was originally developed and funded by the community
then later was proprietarized by a company which happens with permissively licensed
community projects. But in the case where a company develops a software from scratch and
is the sole owner of it, I have no comment about their competitive practices by offering a
GPL version alongside the usual proprietary version.
There is one problematic case that often arises and that is when community developers
decide to submit the changes back, and they do not want their community effort to become
proprietary software. My solution is simple. Fork it. Make a community fork where
copyrights are held by the community. It's that simple.
Whether I think that the nullification clause of copyleft-next replaces the the need of
the previously mentioned forking solution; My answer is no. Because the nullification
clause falls back to permissive, and permissive means anyone can make a proprietary
version of the software, while a community fork would be under copyleft instead which
means that no one can make a proprietary version of the community funded effort.