Im one who cant prevent it using leather and guns without rust impervious finishes. Its also not for lack of taking care of them.
I sweat like a pig, and have worked outside physically, pretty much every day of my life, and worn a handgun at the same time, all that time. The leather holsters I used were not (or ever) cheap holsters, and at the end of using them, in the summer, I was rotating through two horsehide Galco Royal Guards every other day, and neither ever dried out over the summer. Both were constantly wet, and due to that, so were my guns. Not only were they constantly wet, but they were constantly wet with sweat. Once rust started, other than removing it and a refinish, it only continued to get worse, and often at a pretty good rate.
Up until my first hard chrome 1911 and a Glock or two, I carried blued, parked, and SS guns. ALL of them rusted in the summer when using leather holsters, and all did so fairly quickly. All were wiped down daily, and left out of the holsters overnight, and still, continued to rust until they were sent to my local smith and refinished.
Last summer, I tried using a Ruger LCP in one of my Smart Carry's that I used my Seecamps in, and it literally rusted the first day I wore it! The LCP being bigger than the Seecamps, stuck up just enough above the sweat guard back on the holster, that it was against my wet tee shirt. The slide rusted where it contacted it. Ive since switched to a Glock 26 with a properly sized holster, and have not had a speck of rust. The gun often has sweat/moisture on the upper, rearmost part of the slide at the end of the day, and it doesnt rust.
Well, for one thing, it sounds like that holster is telling you something....like lose the jelly roll.
Ive been there too, by the way, although I never had the discomfort.
Ive carried IWB most of my life, fat and skinny, and everything in between, and actually found kydex to be a lot more comfortable than leather. You obviously havent had your tee shirt ride up, and that wet leather holster against your bare skin, quickly rub you raw, and then continue to rub you all day. And to add insult to injury, those chemicals they used to tan/treat it, start it really burning good.
Kydex just slides across wet skin, with no raw spots and no chemicals.
These days, I carry a Glock 17 in a Blade Tech Ambi "Klipt", and it, and the 26 have yet to show a spec of rust. The only time I did get rust on a Glock, was on the slide stop, where the finish wore to bare metal from constant drawing/reholstering. The holster was a Ted Blocker LFI rig. Other than that, dont ever see it.
Ive had really good luck with Blade Tech holsters. Still have the first I bought for my one Commander and with 10+ years of constant, daily use, its still as serviceable as the day I bought it, over 20+ years ago. Thats pretty good for a holster that cost me $50 +/-. Those Royal Guards were $75 a pop, and I was buying a new one every year.
This was the last RG I used. It was in my holster box for a number of years until I sold it on EBay. What you see there, is about a month of daily use before it was taken out of sevice when I got my first BT kydex holster. The tape you see was a laughable attempt at trying to keep the sweat from soaking in, which didnt do a bit of good.
One other thing Ive noticed over the years, is "perspective" varies greatly. Other than the people I work with, most others I know, live, work, and play, in controlled environments, so exposure to the elements, or heavy sweating, is not the norm. Most of them also dont carry a gun "all" the time, which reduces the guns exposure even more. Their experiences have always been different than mine.
Some of us wear our guns 18/7/365, and the guns dont lead a very nice life. More often than not, its harsh, dusty, dirty, sweaty conditions, and thats 12+ hours a day this time of year. Even with a kydex holster and rust resistant finish, they still require constant care and attention.