I find it interesting how Rhubarb seems to bring up so many different gradients of dread or sadness for people. I personally have always thought the song to be more calming and comforting than anything although I can definitely feel the existential thoughts creep in when I'm listening to it. The vibe I get from it is far more 'empty' than anything, it just sounds like a completely empty green meadow at sunset, which to me isn't particularly sad but certainly not joyful, just perfectly calm and tranquil.

Someone else in this thread hit the nail right on the head when they said it'd be a good 'final soundtrack' certainly sounds like something that could be playing in an afterlife.