After reading many Brooks debates I haven't run across thoughts on "losing breathability" after treatment. If one coats the leather with all of this waterpproofing beewax whatever you have to lose Brooks' "claim to fame" breathability...No???? Correct me if I 'm wrong but isn't leather more breathable and less water resistant with oil treatment vs beewax treatment. Oil softens and has some water resistance while beeswax soften(??) BUT has more water resistance thus less breathability. Anyway you shake it, it's all better than plastic. I just have a slow day(after a few months of grueling work). Charlie
Brooks saddles are made from fairly thick leather. Treated or untreated, they're not gonna "breathe" much (defined as allowing air to pass through), except where they are perforated. Being a natural material (animal hide) the saddle will darken as you sweat on it, form to your specific butt-bones and, after break-in, will probably be more comfortable than anything else (IMHO). Plus, they are durable & have class!
The treatment is to preserve the leather by replacing the natural oils (preventing excessive drying & hardening) & to minimize damage from outside moisture. Brooks (& others) don't recommend oils 'cause oil tends to soften the leather more, sometimes leaving it "stretchier", requiring more tightening, etc.
Although some folks like to soften the saddle with oil to ease break-in, the saddle will probably last longer & require less re-tightening if broken-in by riding & then ocassionally treated with Brooks Proofide or something similar to preserve the leather.
We haven't ridden anything but leather saddles for years, so I guess I can't comment on other current saddle materials!