If ME were to be like real life Shepard could've died a while before the trilogy started the more battle's one fights the more likely they'll get killed. Also plot armor is a major factor as to why Shepard survived such situations not skills or training so really skill has little to do with why Shepard has survived so long.
Yeah,luck is clearly the only thing that matters in a battle...
Miyamoto Musashi,Leonidas of Sparta,Alexander Magnus,Genghis Khan,Julius Caesar were great warriors and survived against impossible odds only because they were lucky enough to do it.
Of course they had no training,skills,or even courage enough to face everything and rise,the only reason they survived after so many battles was because of their luck...
Listen,this time for real,some civillians become warriors,some warriors become heroes,and some heroes become legends.That's the way it is.
They had luck? probably.
They died after many battles? Yeah,sure
But they lived long enough to put their names in history,and that was not because of their luck or cowardice.They did it because they were strong and smart enough to beat numerous foes.
As I said,luck matters,but it's useless if you are not prepared enough for the challenge...