Or you could contribute back to the copyleft project (without copyright assignment or relicensing grants) and then you're all on equal terms again: nobody can make proprietary forks.
Why should i take the freedom of making proprietary forks away from people?
My goal with open sourcing my software are to make the (software) world just a little better, i do that by writing the best software i can and hope that as many people possible use it, if i write the best ftp library in the world and you need a ftp library for your proprietary program i hope you use mine instead of an inferior implementation.
In my world view i do not loss anything by you making a proprietary forks of my work (i still have my version of my work with my license) but you and your users are getting a better product.
If your priority is to improve the quality of the software that people use, I don't think you should use copyleft.
If you think that quality is important, but that as important as that is that users should have control over the software they use, as part of the control they should have over their own lives, then I think a copyleft license is a good tool to help achieve that.
My comment was in the context of big company X, "releasing something in a permissive license" vs "releasing something in a copyleft license". Either way I prefer to use, support and contribute to permissively licensed projects.
