I am a long term Brooks user, though I can't match 30 years. A recently bought steel railed B17 seemed to be softer leather than my other, 18 year old, saddle and it has sagged a bit (i'm about 170lbs). Even more recently I have put a titanium railed B17 on my new bike and done about 2500 miles on it. This saddle is pretty stiff leather and has kept its shape totally, it is nevertheless very comfortable.

In my experience, neither sweat nor proofhide makes much difference to Brooks saddle softness.