"If a couple has no common defective gene, they can produce a big healthy population from just two people, right?"
No. As described in the review "The genetic basis of disease" Essays Biochem. 2018 Dec 3; 62(5): 643–723. Published online 2018 Dec 3. doi: 10.1042/EBC20170053
"we now know that, on average, each individual has several hundred variants that are either known, or predicted, to be damaging to gene function, including roughly 85 variants that lead to truncated (incomplete) protein products."
Since a given 2nd generation offspring of a founding couple might have both their copies of a given gene be the exact same version from one grandparent, if that version is defective, both the offspring's copies will thus be defective. This is the consequence of inbreeding and is why for example conservation biologists try to preserve as many different individuals as possible as founders in a captive breeding program.