Like others here, I carry fresh (thick) socks etc.... and that's what I wear for the majority of the flight... a pair of thongs (I know, I know, you call 'em "flip flops.. so have a giggle...

) for bathroom visits....
My flying is mainly transpacific.. and I am NOT leaving my shoes on for flights that long!!!
I do not thin it "innapropriate and rude" at ALL! YMMV of course...
(but I'm still taking them off........