The point of any story is that you aren't going to right about the poor schmuck SM's that die as a scout or the troops that take a plasma gun or dakka gun shell the first time they step outside in power armour. So authors have the right to follow some special cases to make an awesome story.
It's worth noting, as you just said, that Deathwatch also pick the very best Space Marines so you have access to a potential hero. You don't have to make it easy for them either though.
