I'm a total lurker and own only two Hermes items so I don't chime in often...but when I saw this thread, I HAD to temporarily de-lurk!
I have a 21-year old (1989) Rouge H 40-cm swift (gulliver? sorry, i'm not ultra Hermès educated!) birkin with ghw. I live in a cold climate -- snow, sleet, ice, temperatures below zero in the winter months -- and I use it every day and everywhere in the city (public transportation, shopping, travelling, etc.). It sits on the floor when it has to and I don't think twice about it. It's faced the "winter elements" regularly (although I wipe off the moisture when I get indoors). It's even been to five continents and held up amazingly well. No slouching as of yet...
To give you an idea, I'm not a bag "collector". I personally don't see the point of an extensive bag collection -- it doesn't fit my lifestyle or money-spending strategy (I'd rather travel than get another bag) -- so my birkin is the only "handbag" I own. When I purchased it, the idea was "I will spend a high amount once on the perfect bag and use it to death!"
Well, the corners are a bit scuffed and there is some "normal" wear and tear (i.e. scratches), but i have to say that it's the absolute best investment I've ever made. It's functional, classic and all-around fabulous...and looks much newer than even half it's age! I can't speak for other Hermès leathers, which may hold up just as well, but I wouldn't hesitate to get a swift and plan for the "long term"!
Hope this is helpful!