> The two declining authors don't have copyright on the new replacement code, because they didn't write it. So their choice of license can't possibly have any bearing on the license of the rest of the project.

But they influenced the shape of the code.

Imagine if Google loses their upcoming case with Oracle. Any rock thrown into the river will forever taint a codebase unless you can rewind before it was committed or the code is 100% leafy and isolable.