It's plausible in the sense of believable. I did not say it was solid or well-argued. You raise excellent counter arguments. However the point of my comment was

It's plausible in the sense of believable. I did not say it was solid or well-argued. You raise excellent counter arguments. However the point of my comment was there is no need to even go that far, the whole notion of civilizations being carried by shiploads of newcomers is flawed.