Thank you for your feedback, I completely understand your point.
With the current version of Telerik UI for WPF, we provide the IconResource markup extension that allows you to switch icons at runtime based on predefined paths. You can take a look at this article
for more information and the respective SDK example
to see how it works.
Additionally, we're currently in the process of creating a set of flat icons that can be used with RadRichTextBox and its UI and will provide a consistent look for the Office2013 theme. They should be out with the next official Q3 2015 release and will utilize the IconsSources extension as well.
I hope this will suit your needs.
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items