XPS 9315 keyboard design fault - wears away quickly - not covered under warranty

This is the first time I’ve been really disappointed with Dell. Within two years, the coating on the keys started wearing off, so the backlit letters are barely visible. I’ve tried using a felt-tip pen to restore them, but that rubs off quickly too. Any suggestions? Dell quoted around £400 for a replacement keyboard, which seems excessive. I think this is a serious design flaw. Does anyone have advice?  The laptop is still in warranty.