You may say the holster/leather isnt to blame, and you might be able to leave it in a "dry" holster indefinitely, but once that holster absorbs moisture, and gets wet/damp, it is the vehicle that puts the gun into the position of being in that environment and the guns will rust if they are left exposed to it. Trust me, I had decades of experience dealing with it on daily basis, especially in the summers. Leather gets wet, and stays wet, and wet leather holsters lead to rusty guns.
Now, I admit, I worked outside, long days, doing physical things, in all weather, often sweating like a pig, and I carried a gun the whole time. Sweat, dirt, dust, stone dust, etc, all attack leather holsters (all holsters for that matter, but with different results), that in turn, attack the the guns in them. If it weren't for the leather holsters, the guns finishes wouldn't have looked like they did, and wouldn't have rusted.
Maybe if you work in an office and just occasionally wear a gun, in a pristine holster, you have differing experiences.
The best thing to happen to the holster world, for anything being used regularly, especially in real world environments, is Kydex. Hands down. Proof is in the pudding too, as as soon as I went from using a leather holster to one made of Kydex, my rusting issues dropped off about 85-90%. Finishes like hard chrome and tennifer, took that up to about 98-99%.
Kydex also reduces the wear on the finish to just a few key spots, which are generally very small, and often not even noticeable depending on the guns finish. They don't rely on being tightly "molded to the gun" to retain them, like leather, and because of it, you don't have the overall wear and washed out look in the finish that you get with guns carried in leather over time.