If you consider overall design considerations, then why have casters traditionally been glass cannons? Boardly speaking, it's because in a fantasy settings the casters have a monopoly on aoe damage. Once you move to the Starfinder setting it is clear that monopoly is broken, and broken hard. We know there are flamethrowers. Which one has to assume pretty much duplicate the effect of spells like Burning Hands, and can be used by the mechanic class at least, but probably Soldiers too, and maybe others. It would be surprising if the setting doesn't feature grenades (duplicating Fireball) and missile launchers (= meteor storm).
So the question isn't "why do we need Soldiers." but "why do we need casters?". Clearly not for thier damage abilities, which other classes can do just as well. So, for healing, buffing, debuffing and crowd control then. And there is no longer a need for them to be glass cannons.
So, how are classes differentiated? I would suggest that by the gear they can use, with the more awesome tech weapons like rocket launchers being higher tier than a bog standard laser pistol. Tech tiers could be pretty much like spell levels, with the most powerful weapons only being used (competantly) by high level Soldiers, and the most powerful support items being the domain of high level Mechanics.