IANAL, this is not legal advice, etc.

If you disagree that the WTFPL is a useable license, how does forking & relicensing work? Changing the license requires having a license that allows you to do so to the new target license (or ownership of the copyright). But if our assumption is that we reject the WTFPL as a valid license, how can we simultaneously use it to relicense?

I.e., instead of a needing a legal debate about whether the WTFPL allows you to use the code, it would seem to me you trade that for a legal debate about whether the WTFPL allows you to re-license the code.

(At any rate, the upstream relicensed to MIT, and BurntSushi sought agreement from "major contributors, where I interpreted 'major' as '100 lines of code or more.'", so the entire thing appears moot.)

Yes, clearly you can't change the license of a project you don't have a valid license to.