Look at the stain on your shoe right now. The dark patch in the mesh. The grey that sits in the weave no matter what you put on it. Every time you scrub that stain with a brush you are not lifting it.
You are pushing it deeper into the textile fibre, past the surface, into the foam underneath, into the lining.
Until it is so embedded that no surface cleaning method can reach it. Until you give up and replace the shoe. Not because it was worn out. Because it could not be cleaned.