Lou Reed will go down as one of the most influential musicians of all time, and it is easy to see why. His lyrics pushed the boundaries and exposed its listeners to themes uncommon for the time. His influence to the music world is wide spread, and that many artists cite him and the Velvet Underground as an influence. Lou Reed definitely deserves to be ranked among the best.
Since we have no way of peering into the future, it is impossible to determine whether Lou Reed is the greatest musician of 'all time'.
Furthermore there has been no attempt to explain exactly what criteria must be met to become the 'greatest musician'. The question really does not pose a question, but merely indicates someone's feelings toward that musician.