It's like marrying a beautiful wife, stripping her of make-up and keeping her in the closet.
But without joking: Of course it is unwise to touch a holster that shows no signs of deterioration, simply keeping it clean and out of harms way is the best approach.
That said, there are thousands of holsters that are not first-rate, but can be touched up to a more than decent condition with the help of some minor detailing work.
Others are just too far gone, and any preservation work done on those is just delaying the inevitable anyway, but they make good test beds for the better ones. I have examples here of holsters that were ruined by previous owners, turned into some sort of fat, slimy, mushy lump of leather as a result. One 1936 holster had to be soaked for many weeks, just to get the most of whatever preservative the former owner used out of it. The oil is still seeping out, almost 5 years later....
On the other hand I've seen preservation methods used on WW1 navy holsters that were pulled from a WW1 shipwreck and it is quite stunning to see how the leather surived this ordeal.