What are you talking about? I don't know where you live or what project you are referring to, but it's not possible to retroactively remove someone's rights in the American legal system by publishing a new license. If you committed code under a MIT license without signing anything else, you don't lose anything when later versions are published under BSL, AGPL, or whatever. The fact that the MIT license arguably allows willy nilly relicensing by anyone should be seen as a flaw with that license, not BSL. Also the context of this conversation is what license is appropriate for a new project. It makes less sense to apply a BSL to a MIT licensed project that has been around for a while than it does to start off with a BSL.

There is less of a difference between BSL and MIT than between any GPL license, as I'm sure you are aware. If I'm giving away software including source in the open, for free, while allowing redistribution and modification, then I'm not sure how to convey that without offending patrons of the OSI and FSF. Not feeling like walking around on eggshells to give things away that's for certain.