Kydex will wear the painted finish off of Glocks (and other types of finishes as well as scratching hard chrome) very easy. Back when Glocks had a different tennifer process with a dark charcoal color thru-out, a file wouldn't scratch them. My pistols with the Melonite QPQ finish have held up for years while being drawn from a Kydex holsters.
On another note, slide wear shows character. I have a Glock 35 that have over 96,000 rounds thru it and has been drawn from a Blade-Tech (Kydex) holster for 8 years. A lot of the slide is showing the silver tennifer finish, I'm proud wihen I display it!
IDPA - No excuses, No sight pictures, No gaming, No rehearsing. Shoot now or go to the back of the line!
"Speed is fine, accuracy is final" - Larry Vickers