I personally have had several babies born that would not have gotten out of the sack had I not been there- the sacks still had quite a bit of fluid in them, and the one baby got a good nose full of it before I could get it out. On two, the sacks were very thick and I could not even force my thumb through them (and I am not a weakling) so the baby would never have gotten out of that.
My first foal did not get out of the sack and died. He was born alive and we could tell because of the position he was in. No one was home to help. He was extremely tiny and just could not get out. (his dam was only about 30")
Our vet goes with the theory that they suffocate, not that they are already dead. The ones I have had that aborted because of an already dead foal did not come out on their own, the sack was already torn, etc... a whole different ball game. Not that something could not happen and one may have died and came out in the sack and all. The mare who lost her foal this last year on Xmas Eve did not abort it still in the sack- I have never had that happen and I have lost about 5 due to aborting (had one mare that was chronic).
Every aborted foal I've had, had to be pulled due to being in the wrong position. If the baby is already dead, (due to a cord twist or whatever) it cannot move around to get into the birth canal to be born in the correct manner of front legs and head first, etc... and I have never had one that was in proper presentation position die during birth and still come out in the sack.