Earlier quoted context omitted.
A knock-out study will work as well as the precise definition and measurement of the phenotype you're looking for will allow. The point you're making is well understood by geneticist. There's about 22K genes in the genome, if your typical genetic screen returns a list of a 0 to 1K positives, no one is claiming that the 21K genes are absolutely uninvolved in the phenotype.
i have read many studies that claimed a gene had no function in a pathway after performing knockout and observing no effect. there are many great research papers, but there are also many flawed ones.
Could you point to one concrete example where you believe a paper is flawed in its conclusion, where they've claimed a gene has no function in a pathway under the given experimental conditions, after performing a knockout?