That's not suppose to be the real Superman aka Pre-Reboot Superman, but the Kingdom Come version.
I think when this future Superman was introduced, there was
something in mind for him, but I don't know if we'll ever find out what it was. The whole "Whatever Happened to the Man of Tomorrow" reference reads to me like some random thing they pulled out of their @ss
<delete><delete><delete>hat, like most of Loeb's ultra-cool-ideas-from-nowhere that are just tossed in with no rhyme nor reason. This ending felt a lot like the RTK ending.
For those who don't mind
SPOILERS, here's the page in question - the final page of
Superman/Batman 18. If you do mind spoilers, just close your eyes.
For the record, I thought this was fantastic, as was the whole Adult Legion thing. Kudos, guys, and thanks!