The real problem for 5e casters is the longbow. Back in 3e, long range spells were true artillery, with ranges in the 600+ feet or more.I'd hope that 4hp commoner fodder might at least bee given some training. 11 hp guards would certainly have a different standard but they are still a way short of rarer 52 hp knights or 58 hp veterans.
The reference I saw in Eberon talked up magic.
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Certainly the problem for AoE casters in D&D warfare is that troops may not clump together.
In 5e, you generally get 150 feet for spells like fireball, versus 600 feet for the longbow.
So “realistically” any mage squad would have to be well protected, likely moved in some kind of reinforced box or something with just an arrow slit opening so they could cast magic without being taken out from a much longer range.
So while yes casters would certainly influence the battlefield it’s not quite the same as older editions when magic ranges are much longer than they are now.