Nature does have to play along, because whatever the correct theory of physics, it has to be logically coherent. Either everything is quantum, or nothing is. G

Nature does have to play along, because whatever the correct theory of physics, it has to be logically coherent. Either everything is quantum, or nothing is. Given that we are pretty darn sure that most of physics is quantum, we're pretty sure the answer is that everything is quantum, not that nothing is quantum. There are different ways of formulating quantum theories of gravity (in fact, you can derive the Einstein equations fairly straightforwardly by trying to formulate a quantum theory of a massless spin-2 boson). We don't know which of those theories is correct yet, but it's not as if there's no way to quantize gravity.