I dislike the "open core" model as well. However, I don't see anything wrong with the copyleft, "release your source too or pay us" model. That provides very similar results: fellow developers can collaborate on the source, and companies building something they don't want to share can pay for an alternative license/exception so they don't have to. But instead of using a proprietary license to do so, that model uses a standard FOSS license, and works with the rest of the FOSS ecosystem.