Pretty much this exact question came up a while back, and u/Iphikrates and I both have written extensive answers in it. I encourage you to read it.

You'll find you're overestimating the lethality of ancient battles. Not that many people died, though a heavy toll would fall on the front rank.

Your supposition about cannon fodder is also incorrect. On the contrary, people tended to put their strongest, bravest and best equipped and armoured warriors in the front row. Read the linked thread for details.

One thing that wasn't addressed there:

Roman Auxilia were by no means cannon fodder. They were valuable, professional, full-time soldiers of the standing army, and by no means cheap or easy to replace. Fully half the Imperial Roman army (1st-3rd century A.D.) consisted of auxilia, including almost all their cavalry and skirmishing troops, though most auxilia still served as heavy infantry.

Auxliary infantry were paid slightly less well than legionaries in the Principate, and certainly had a lesser social status. (187.5 denarii a year instead of 225, under Augustus) However, the Auxiliary cavalry of the elite Alae (wings) were actually paid more than ordinary legionaries, even if they weren't citizens. (262,5 denarii) Just have a look at this silver cavalry face mask that was very likely worn by one such auxiliary cavalryman during the battle of the Teutoburg Forest. That's not how cannon fodder is equipped.

The notion that the Auxilia were seen as more expendable comes, as far as I can tell, from a line in Tacitus' description of the battle of Mons Graupius:

He arrayed his eager and impetuous troops in such a manner that the auxiliary infantry, 8,000 in number, strengthened his centre, while 3,000 cavalry were posted on his wings. The legions were drawn up in front of the entrenched camp; his victory would be vastly more glorious if won without the loss of Roman blood, and he would have a reserve in case of repulse. — Tacitus, Life of Cnaeus Iulius Agricola, chapter 35

As I said, Auxiliary infantry did have a lower status than legionaries. This deployment, moreover, makes tactical sense: having your steadiest troops in reserve. But this does not mean the Batavian cohorts were cannon fodder. On the contrary: going by Tacitus' accounts, the Romans only suffered 360 dead out of 11000 auxiliaries, including the (Roman) commander of one of the cohorts who had been reckless. 3% casualties might seem very low, but it's actually right in the middle of the typical 2-5% range that gets reported for ancient battles.^* (Casualty figures for ancient battles are notoriously unreliable, but it's usually the other sides' casualties that get exaggerated.)

For more details, see Adrian Goldsworthy, the Complete Roman Army

^* (edit) NB: This is the amount of casualties for the victor, or the amount of people who would die during the actual fighting. As explained below, many more people would die after one side broke and ran away. The majority of killing would take place at this point in the fight.

I've also got a follow-up question. Since casualties were usually fairly low in a comon setting (by that i mean, with simpler tactics than Cannae), what do you think would have been the most general factor(s) for victory? Exhaustion? (Didn't see you had jind of closed the topic, sorry. But I don't think this question is big enough to deserve another topic tbh.)

I'd like to piggyback on this thread, what about firearms were being introduced and all the way up until we abandoned formation battles. Wouldn't a soldier say in the American Civil War that was in the front line the most likely to be shot? I remember in boot camp we were doing drill and marching practice and they said it was a tradition from when battles were fought in formation and I was always in the first squad, just thinking that I'm glad we don't fight this way anymore because I seriously don't think I would have the courage to stand up and play an extreme game of chicken. In this regard, wouldn't the men in the front lines almost be guaranteed death or injury?