Simply put, what we humans don't know can doom the turtles.
We don't know where they go during their "lost years", so we don't know if actions we are taking will endanger that habitat.
We don't know how long turtles take to reach sexual maturity, or how long they remain productive, or how often they reproduce. We don't know much about the distribution and range of turtles. We don't know how large the populations are. We don't know how many dead turtles can be attributed to fishing practices, legal or illegal. We don't know what the survival rate of hatchlings is once they are in the water, although we're pretty sure it's low.
Like it or not, humans still must make judgments about the number of dead turtles a population can sustain. In some places, turtles are still legally hunted. In others, there is the issue of what is an "acceptable level" of incidental kill before some action is taken. Obviously, we can't make these judgments if we don't know how quickly the population reproduces, or how well it does after reproducing.
There is a frighteningly real possibility that some decision, made in ignorance, can doom a whole turtle population, perhaps an entire species. Of course, we won't know it until they simply stop showing up.