If you (and everyone else who've made the same argument) substitute "Syntactic Sugar" for "Abstraction" and "Turing Complete" for "Relational Model" I think you

If you (and everyone else who've made the same argument) substitute "Syntactic Sugar" for "Abstraction" and "Turing Complete" for "Relational Model" I think you've just made the claim that every application can or should be written in Brainf*ck or one of the other Turing tarpits. Just because a model is mathematically proven comprehensive, (I'm not 100% sure if relational is but that doesn't matter) doesn't mean it's the most natural way to express/solve your problem. And really, that is the only thing that matters. At least when comparing models and not implementations.