The question is then to what extend that particular system is an accurate reflection of reality, and to answer that question you would have the analyze the delta between them, both in terms of how it was initially conceived vs how it evolved.
In the case of the US, at a very basic level for example the founders did not envision the US as a multiracial society or that everyone, including non-landowners could vote. Also that the US constitution itself looks very different today in terms of its amendments and more importantly the case law and jurisprudence around constitutional law which is de facto the constitution of the US. I don’t mean that these are right or wrong necessarily, just that the initial conception is very different.
Basically I think you are right but that also the basic premise of liberalism and the liberal system even in its initial conception is flawed and therefore doomed to fail.
I do think fundamentally liberalism does assert a view of human psychology and sociology that assumes that properly organizing and orienting the human mind is at least 3 orders of magnitude easier than it really is. If you actually study anthropology, like the anthropological studies that were done in the 19th century and if you study depth psychology which heavily makes use of those anthropological studies, you find a very terrifying and very chaotic and complex picture of the human psyche, as revealed in the religions, mythologies, and rites of pre-civilization peoples which liberalism has virtually absolutely no awareness of.