How does a narrator's trustworthiness make a hadith true when he could have got it from an untrustworthy guy?

Way I see it, you have a load of contemporary narrators (whose familiars are asked about in order to determine trustworthiness) that ultimately leads to an old narrator that narrates from someone who was an elder of his. The old narrator's familiars are asked about the reliability of that elder of his and what their elders said about the reliability of HIS elder i.e. tabi'un that got it from the sahabi in mid-20s when the sahabi was in their mid-80s. Coz this adds up to the time between Rasulullah (SAW)'s death and when Bukhari (RA) started collecting hadiths between age 16 and 32; during which the old narrator would be aged between 65-80.