This fannish theory was that the reason the Dursleys spoiled Dudley was that they were deathly afraid that he would be as stricken by the news that Harry was a wizard as Petunia had been, by Lily, so they were trying to beef up his confidence before then. (Humanizing them far past their comic role in the actual books.)
For some reason, I remembered that theory today. And poked at it a bit. And realized that the really, truely, honestly most horrible thing that could happen then -- or even without this theory -- would be if Harry turned out to be a wizard -- and so did Dudley.
Just imagine the scene where their boy observes that one of the letters was addressed to him.
Though actually, it wouldn't work. Dudley is too limited to type to work as a character for the bulk of a book. But with the serial numbers filed off and the Dudley equivalent getting some actual character, that would be an interesting plot twist. One wonders if the Dursleys equivalent in such a story would not just turn both children out of doors and never deal with them again.