I honestly think Syl will ultimately make it to that point; she's already the person Kaladin relies on most, and it's implied that as their bond progresses, she grows more emotionally mature (as seen in the way she regresses when their bond is nearly broken in WoR). I think that by the time Kaladin has completed the fifth ideal, Syl will be fully equipped to handle him, even at his lowest. Also, let's not forget that through all three 1000+ page books, Syl is one of the only people, and the only one who's still alive, that he's shown a true, genuine smile to.