"Scientists and economists advise politicians" is not quite the same as "scientists and economists indirectly control policy." In practice, in the US, most advisors who are not also politicians are pitted against an equal and opposite party and then ignored. That is not universally true and it is not the only problem with the US government by a long shot, but it is a significant effect.
As for the Chinese government, you were more or less right until relatively recently. They were fairly successful too (which is not the same thing as being nice, of course).
In practice, in the US, most advisors who are not also politicians are pitted against an equal and opposite party and then ignored.
You expect this in equilibrium regardless of how far you are from the counterfactual with no economists, so this is not evidence either way.
or: Why Everything Is Terrible, An Overview.1
It sounds like a theory which explains too much. But it's not a theory, hardly even an explanation, more a pattern that manifests itself once you start trying to seriously answer rhetorical questions about the state of the world. From many perspectives, it's obvious to the point of being mundane, practically tautological, but sometimes such obvious facts are worth pointing out regardless.
The idea is this: The subset of participants which rises to prominence in any area does so because its members have traits helpful to becoming prominent, not necessarily because they have traits which are desirable. Thus, without ongoing and concerted effort, a great many arenas end up dominated by players employing strategies which are bad for everyone.
This comes up again and again: