What is the rationale behind gravitons?

[Eelco]

We know that energy stored in a gravitational system can be converted to energy stored in other kinds of systems. all those other kinds of systems require that energy be quantized. If energy in a gravitational system were not quantized, then how could it smoothly flow into another type of system which accepted energy in packets? So this is why gravitational energy must be quantized.

Yup, that makes sense to me.

Yet in general, I don't think it is a good thing to get too hung up on things like invariants, or even conservation laws. Yeah, they seem to hold. As far as we can tell, which is only to limited resolution.

If your model can explain all observations, it is good to me. To convince me it can, you need to actually compute stuff with it, and compare it side by side with observations. To me, juggeling mathematical theorems is a means to an end, not a goal in itself.

SR and GR, space-time, continuums, manifolds, dimensions, and Newtonian mechanics are descriptions of the large-scale behavior of many individual machines (particle interactions). You are right, there is no reasonable merging with the behavior of individual particle interactions for any of those large-scale theories. Scientists continue to erroneously presume theories developed solely to describe the average behavior of many simple machines will also be the founding theories in describing the behavior of each of those machines. There is no reason to believe that.

Yeah, we completely agree here.