What I would urge, in advance this time, is you don't rescramble the URLs such that existing links into your existing archive get broken.
In moving to the newer (Java based?) system, all the URLs inherited from the Swarthmore College era were destroyed. Asking archive.org to pick up the slack is I think asking too much.
As far as the size of the subscriber pool being small, the major contributors being few, the content having R or MA (mature adult) aspects, that's a common fate of many eGroups, like becoming a brown drawf or red giant. Think of Conway's 'Game of Life' -- there're many stable outcomes.
In other words, I don't see why Math Forum staffers should busy themselves worrying about the content over much, or about the complaints from those who've resigned in protest. There're any number of other lists to sample. math-teach simply is what it is, so what's the problem?
That'd be my attitude, if the President of Drexel University (color me "liberal arts"). But I'm not, and you may have instructions to "clean it up" or whatever (I'm so sorry if that's true -- can't be a fun job).
In any case, back to the above request: please don't break the URLs, even if you retire the list from active duty and ban any further posts thereto.