If they had gone generational though, with about 150 crew aboard, would there be enough genetic diversity to avoid inbreeding? Was the male to female ratio ever specified?
These people are not hillbillies, unless you mean that what if it was a longer trip than 70 years?
On a seventy year trip, maybe the great grand children were being born.
30 breeding couples safely produce 15 genetically similar groups of offspring, who safely produce 7 genetically similar groups of offspring, who safely produce 2 groups of genetically similar offspring who safey produce children who have to fuck their cousins.
4 generations, by my count, before these people get into trouble, which they would take 350 years to arrive at in the good old fashioned way.
If they allow cousin boffing from the beginning, it will prolong the point before obvious problems show up.
If they insist on key parties, and make sure that evey woman has as amny baby dadies as possible and not the same baby daddy twice then they might even be able to last perpetually.
Chakotay's estimation of needing crew replacements in 30 years is becuase he's lazy and wants to retire. In all honesty none of the human crew are going to start dying of old age for at least a century or a century and a half, using mcCoy as a yardstick.