Unless you had a super thin wire edge that could fold over "Kydex" isn't hard enough to dull a blade (metal vs plastic). Rubbing is a different story, Kydex still isn't hard enough but the debris, dust, dirt etc. that gets into your sheath can definitely scratch your blade..,. two totally different issues though since one is just cosmetic. It really does vary from maker to maker and not all "Kydex" sheaths are equal! I remember seeing a "kydex guy" boasting about how much detail he was getting in his sheaths and he showed the imprint of the esee logo on the inside of his pancake sheath, it was stupid conclusion to come to... that much detail means there wasn't enough clearance and that imprint will make contact with the knife every time it goes in/out of the sheath so be mindful of the "geniuses", lol.
Kydex fact #27: "Tape" is not enough
There is one famous issue where "kydex" was dulling sheaths and it started a lot of problems/concerns for sheath makers because people started asking about it-the culprit was the material itself and I'm talking about Kabar Knives (Becker). The BK11 & 14 sheaths had a fiberglass resin and they weren't properly designed to allow enough clearance for a blade to pass through... they still aren't but they solved the problem by replacing the fiberglass material.
Bottom line is there are a bunch of Kydex sheath makers with a lot of "participation trophies", which knife are you looking for a sheath for? I'm not asking for myself because I only make sheaths for ESEE Knives but I might be able to point you in the right direction.