Did you RTFA? The whole point is that it IS legal to change your mind later, and no amount of promises, or guarantees, or written contracts can change that.
Ok, so I RTFA, and I see "One right that all creators have is to undo copyright transfers and licenses after thirty-five years have passed, under some conditions." [...] "Copyright termination means that any license, including a perpetual public license, can be revoked." But that's not what I'm talking about. I'm talking about making a promise not to sue for use of the work, which is not the same thing as offering a license for the use of the work. If you make an explicit and public promise not to sue, separately from any licensing, that's different from offering a license and then revoking it later. Licensing something for any use is not the same as placing it in the public domain, and promising not to sue for any use seems the only way to effectively actually do that otherwise.
This is a threat to the GPL, the MIT, and other [F]OSS licenses. But it's not a threat to the public domain.
I do think that we should have a public domain registry, where we can explicitly give up our right to a work in perpetuity. But why should we need one? We already have a legal concept which should permit accomplishing the same thing.