Oh, and sorry for the necro-post (15 years late! Woo!)
The fact that it's very short isn't really a problem as it's a fable not a novel but the "motto" at the end - "No one remembers the singer. The song remains" isn't really the one I'd expected, it seems only tangentially connected to the main theme of humanity defying fate/the gods.
The book's final line puzzled me a little, until I thought: "What would people on the Disc say about Cohen, now that he's no longer 'there'?" and immediately I thought of the answer. Nobody would remember who the minstrel was - not even the minstrel knows what his name is (he tells Evil Harry "I'm just the singer"), but the songs and stories about Cohen and the Horde will be all that's left.
So in fact, the theme of the book isn't all about humanity defying fate/the gods. The theme is: "What is a hero? What is heroism?"
Consider: towards the start of the book, Cohen is "rememb'rin'" some poor bloke who was buried in the snow. He doesn't know anything about him - his name, if he did any mighty deeds, etc. - but he can still
remember him. At the end of the book, that exactly happens to Cohen and the Horde.
There are also comments throughout the book about the nature of heroism, especially between Vetinari and Mr. Betteridge of the Guild of Historians. (He is named after Pratchett’s own former school history teacher, Stan Betteridge; see
this article from The Telegraph. and
this one from the Bucks Free Press.

Two other characters, Ponder Stibbons and Evil Harry Dread, are also cheekily named after former teachers).
There's also a much more obvious discussion about heroism when Carrot confronts the Horde, culminating in Cohen's line: "To die for thirty dollars a month, a man has to be either very, very stupid or very,
very brave..."
Lastly: now that Terry's been gone for over 10 years, the line about the singer and the song is now much more poignant. How long, I wonder in my more pessimistic moments, will it be before Terry is forgotten?
Which takes me back to favourite illustrations.

Mine is actually two - it's sort of the Before and After pictures of the minstrel. That picture of him in rags sitting on the rock playing his homemade lyre is excellent in my opinion.
Agreed. I also enjoyed the Before and After pictures of the Discworld (after the loss of magic). Is it a stark reference to what the Earth might be after a nuclear war?