You can revoke a license at will, but I'm not aware of any case where someone took something that was open source and successfully retroactively closed it. You can close your future development but if someone takes the old code under the old license and continues work on it, I can think of no case where that was stopped, or even challenged.
So it seems to me there is a real difference here between licenses that proactively are clearly giving you some sort of rights, even if what those rights are may be up for debate, vs. the WTFPL license, which is still technically probably establishes that the author meant to grant you some stuff, but it is not terribly clear what.
> You can revoke a license at will, but I'm not aware of any case where someone took something that was open source and successfully retroactively closed it.
Revoking a license means that permissions previously granted are no longer applicable to any acts taken in the future, so no action requiring a license under copyright (such as copying) would generally be permitted (promissory estoppel protects, to a limited extent, those who initially relied on a promise that the license would not be revoked before it actually was revoked, but generally only to the extent that they had reasonably expended resources in that reliance and would be suffer unjust loss due the violation of the non-binding promise.)
> You can close your future development but if someone takes the old code under the old license and continues work on it, I can think of no case where that was stopped, or even challenged.
Thats because no one has ever actually revoked an open source license on code in significant use, rather than merely not offering it for future versions. And there's a good reason: most people offering open source licenses, even if they change their mind on a particular work, want open source to keep working, which actually relies very much on the shared belief that gratuitous open source licenses won't be revoked, even though the law of gratuitous licenses clearly allows that.
If it ever happens (and courts don't just completely rewrite the law on gratuitous licenses in defiance of precedent to save open source), the whole illusion of safety comes crashing down.
So it seems to me there is a real difference here between licenses that proactively are clearly giving you some sort of rights, even if what those rights are may be up for debate, vs. the WTFPL license, which is still technically probably establishes that the author meant to grant you some stuff, but it is not terribly clear what.