DonIago said:
Did you go into any Trek series expecting that at some point the principal characters would be killed off?
Perhaps it's not very realistic, but it _is_ the nature of television. The same way the Enterprise and DS9 aren't going to be destroyed in the course of the series...most likely.
Also, in most cases they _did_ know the risks. They may be scientists/engineeers, but they're also Starfleet officers. I doubt you can get through the Academy without hearing all kinds of tales about how things went badly for your predecessors.
Plus...hello, they're on a spaceship. That in and of itself is a risk.