There's a time limit as Clark doesn't want to be stuck as Lex Luthor (despite being married to Lois), which he will be if Lex-Superman stays in the Kryptonite cage too long... and dies. However, if Lex-Superman escapes, he could then always expose Clark-Luthor as the evil boss, sending non-powerful Clark-Luthor to jail for Lex Luthor's crimes, while Lex-Superman is free to woo a heart-broken Lois, while being an all-powerful hero, who can literally take over the world.