I used to do custom holsters, and the LAST thing you want is a soft holster.
One reason older guns have little or no finish left is because they were carried in the "one size fits all", soft, flabby holster of the day.
Leather, and ANY other holster material will abrade the gun's finish, and there's little that can be done about it.
What causes the wear is contact between the holster and the gun. The more contact there is, the more finish can be worn off.
Holsters are abrasive, and a soft holster which contacts the gun over a wider area will wear more than a hard, stiff holster that only contacts some areas.
A soft holster allows the gun to move and shift around as you move.
When the holster gets dirty inside, it is in effect, turned into a sandpaper lined bag.
MOST holsters are used far past their service life.
Although a holster may LOOK virtually brand new, when the holster has stretched out enough to allow the gun to start shifting, it's shot, and should be replaced.
Plastic holsters cause faster wear to the finish, BUT, the wear is ONLY on the contact areas, and in a good model it doesn't wear anywhere else but on those areas.
Over time, a plastic holster will cause LESS wear than leather.
Leather and nylon holsters wear over a wider and wider area as they stretch with use.
Some holsters are WORSE than standard leather models.
Holsters lined with sueded leather will wear the finish faster due to the soft suede making more contact, AND the tendency for the suede to collect dirt and become even more abrasive.
The worse holsters are the "one size fits all" holsters made from cheap, flabby leather, and holsters someone mistakenly over-treats with leather softening products.
A good holster, in serviceable condition should be fairly stiff, and the gun should "snap" into place. Once in, the gun should NOT shift or move.
When the holster is no longer in this condition, even though it still LOOKS great, it's worn out.
Last, contrary to popular belief, leather cannot be "shrunk" back to a good fit.
The ONLY leather that actually shrinks is Rawhide.
If your new holster is TOO tight, either break it in by wearing it around the house for a few days, practicing your draw (which is a good idea to train your muscle memory) OR wrap the gun in a layer of wax paper, put it in the gun and leave it in over night.
If it's still too tight, add another layer of wax paper and repeat.
Soaking a new, quality holster in water or alcohol is NOT a good idea, since this usually stretches the leather TOO much, taking a considerable amount of service life out of it.
Bottom line: If you don't want your gun's finish worn, don't ever put it in a holster. Better still, put it in a glass case and don't even touch it.