They were not marching to certain death. Until world war one, most soldiers that died, died when they were attempting to run away.

The hit frequency of smoothbore muskets were extremely low, which is why volley fire was needed. The real struggle was who would keep the line the longest and not run. The paradox there were that if you ran too early, you would get away, but you risked being cut down by your own officers for desertion or "fleeing the banner" - if not cut down, your name would be taken down and you could face court martial after the battle. If you ran too late, you would almost certainly be cut down by pursuing enemies.

Thius is why you often see well-disciplined troops take one or two salvoes from the enemy before firing themselves - getting in closer meant that your volley would do more damage, bot physically and to the enemy morale, and showing your discipline under fire established moral superiority and unnerved the enemy, increasing the chances that they would be the ones running first.

For example, in the Battle of Fraustadt 1706, the Swedish army of some 9 400 decisively defeatet a combined Russo-Saxonian force of 15 000 - the Swedish forces suffered 452 dead, or 4,8%. The Russo-Saxonian forces suffered 7 377 dead, or 49,2%.

Considering Swedish forces of the time deployed in formations four ranks deep - even if every Swede that died were in the frontline (which certainly was not the case), that means that 19,2% of the frontline died. Not very good odds, but not "almost certain death" either.

Being in the frontline did not mean certain death. However, routing often did, especially if you were among the last or first to run.