I won't comment on "the-itis" because I'd never heard of it, but it does seem like in some cases people are taking a phrase or word that no one has known anything offensive about for a hundred years, and basically researching it back into offensiveness.

As Joy Silver said above, "rule of thumb" (allegedly) used to mean the maximum width of the stick a man could beat his wife with. I'm not sure what the historical evidence is for that, but either way I don't care. I'm still going to use "rule of thumb" the way people have used it for decades or longer. I am not going to scrutinize its origins and work myself up to being offended by it.