Try saying that around a geneticist :( most i know consider mendelian genetics to apply to everything and nearly all traits/phylogenies are considered monogenic.
You hang out with the wrong geneticists. Most do not consider most traits to be monogenetic. It’s just vastly easier to focus on them when doing research. But the field as a whole has long since accepted that most interesting traits aren’t Mendelian. The whole field of GWAS and (e)QTL studies is founded around that recognition, and virtually all modern genetic models assume at least an additive effect of (known and unknown) variations as contributors to a given phenotype (and have, for a very long time).