All you 'Til I Die voters you gotta be kidding - there's nothing sad about its melody/sound/lyrics whatsoever. The extended mix even less so.
Lyrically the song is about being a speck of nothing in a world in which you have no control over. To some that feeling of insignificance could be inconsequential, to others it's truly sad....there's nothing wrong with either perception.
As for the overall mood of the melody and instrumentals, the Desper mix does a perfect job of creating an atmosphere of void. Each note played on the vibraphone rings out beautifully then vanishes into emptiness (which could be construed as sad - depends on the listener).
Woodstock mentioned earlier that the outro sounds more like a mantra, the narrator comes to grips with the reality of life and finds peace. The melody/chord progression in this section sounds very uplifting, so one could definitely argue that the song's coda is meant to be uplifting. But this is Brian Wilson we're talking about here - his music sometimes never reflects the nature of the lyrics....thus allowing varied interpretations for each listener to choose from (which is part of the brilliance of Brian's songwriting). There is no right or wrong answer, it just depends on the listener.
Maybe that "space" is built-in to allow your mind to wander. They say that the kids who "stare out the window" in school are often the most creative because they find places to let their minds wander, in an institutional setting, which often fosters creativity. My take is that Brian realizes that we are a small spot in the larger cosmos, regardless of our stature (whether worldly, high-profile, or not.)
And, I love the Endless Harmony Alternate Mix (Mr. Rockin' Desper!) with its deliciously long and undulating melody section, and I agree that it is a lyric that is a "coming to terms" with life...
Happy Friday the Thirteenth!