Kiefer (2014) claims that,
"It is, in fact, the superposition principle that points towards the need for quantizing gravity."
Moreover, Kiefer (2009) stresses that,
"The only assumptions are the experimentally supported universality of the linear structure of quantum theory and the recovery of general relativity in the classical limit."
But why is the linear structure of quantum mechanics so important? Don't we already know that nature is non-linear (for instance, interacting field theories)?
If we do accept that the superposition principle is not really universal, how do the above arguments for quantum gravity hold?