It's less the difference between a 1/3 and 1/2, and more a load of other abilities which set them apart.The only non-setting specific archetype 5E can't currently do using the existing classes is the psion. Psionic subclasses work well enough for a few subtypes, but the core concept of a psion doesn't fit into any of the classes neatly enough. I wouldn't mind a true half-caster warrior-mage, but this is a nit-pick difference between a 1/2 caster and the 1/3 caster eldritch knight.
As eldritch knight is tied to the fighter power budget, all the things which set the gishes apart in prior editions have had to be axed, nerfed, or delayed to high level.
Like for example due to being a subclass it has to just use the wizard list, which is awful for martial combat blended with weapons. A single class swordmage would have its own list, full of spells which it can integrate into its weapon strikes. Look at the paladin and ranger lists, and compare them to the cleric and druid lists. The theme is the same, but the spells have a different focus.