So when you go to delete a material Sketchup asks if you want to replace it with the default material. How hard would it be to give the option to replace with another material? It would make my life so much better.
there’s a plugin “material replacer” that will replace a material with another. Might not solve your exact issue but might help?
If you have V-Ray, the Asset Editor in V-Ray for SketchUp has this feature. Handy because it can replace materials inside groups and components in one go.