A friend of mine had an argument that towels are supposed to be used for nothing more than two years but he said ten if possible. Won't the towels become a habitation for germs in ten years time?
They wont if laundered properly, I'm sure hospitals use sheets etc.. for surgery for longer periods.
However, I agree with you, my husband wants to keep hideous, ratty towels like they are a treasure- they may be clean (and I know because I wash them!) but it looks like me are living in a crack den.
I say two years tops- and they are always on sale so get a new set when you can