I've gotta ask: How many recruits do you think a T5 unit should be able to kill (on average) if not five. Three, four?
Though legionaries are indeed stronger than most T5 units, I don't agree they are worth the money they cost. T3 troops will get you similar results for less time, upgrade money, and wage money investment (and, in the case of cav, horse upgrades). T5 is not a large increase in effectiveness relative to T3.
And cost effectiveness is just one factor for why T5 troops should perform very well against raw recruits. Realism and the feeling of progression are two others.
I've given historical examples of battle hardened, well-trained, well-equipped soldiers inflicting disproportionately high casualties on inexperienced, poorly-trained, poorly-equipped soldiers. So if anything, high tier melee units' performance with their quality weapons, armor, and training is
unrealistically weak against raw recruits who have a weak little weapon, no armor, and no training or experience of war.
Progression is the other big thing. After putting work into training your troops, you want them to feel powerful, to be able to easily crush the weakest enemies in the game.
In Warband, each T5 Swadian Knight could kill 10 T1 Recruits. That's obviously way too powerful, but it did feel satisfying. A Legionary being able to kill 5 Recruits would be half as strong as that, so it would be a midpoint between Warband and the unsatisfying weak elites we have now.
Yeah, I should have mentioned that ranged fighters are an exception, especially ones like Sharpshooters.