Does a sharpening steel ever need to be replaced?
Standard grooved metal steels don't ever really become unusable.  The ridges will get dinged up over time with abuse, especially with cheap steels that are not of good quality; however, I've seen some seriously (ab)used steels and they are still quite usable despite looking like Rocky at the end of a fight.  In the worst case, the steel will become smooth... which doesn't hinder its ability do its job and align the blade edge.

Smooth (ungroved) metal steels cannot ever wear out, since they're just a smooth piece of metal.  

Ceramic sharpening "steels" will never wear out, but the surface can get clogged with removed metal particles.  This can be cleaned out with a scouring pad to render them as good as new.  If banged against things, the brittle ceramic can chip permanently, but that is not part of normal use. 

Diamond sharpening "steels" are the only ones which ABSOLUTELY WILL wear out over time; the abrasive is made of tiny diamonds embedded in the surface, which will be dislodged by normal use.  Eventually enough diamonds will be scraped off that the tool ceases to sharpen.