I agree that there are too many unit classes, and they create large gaps which leave units without upgrades for entire eras. A few examples:
- Musketmen upgrade to Infantry, and have no intermediate Industrial step. Where's the Rifleman?
- Horsemen don't upgrade till Cavalry enters the scene, (at least?) 2 eras after the former's inception.
- Knights don't upgrade at any point until the Modern Era, once Tanks are unlocked. They essentially go by a thousand years without an upgrade. A Cuirassier would be a good middle ground.
- If I remember correctly, the Bombard upgrades to Artillery, left out to dry for the entire Industrial Era.
- Quadriremes only upgrade once the Frigate appears, like 3 eras later.
The list goes on. Too many gaps, all over the place. It's one of the reasons the AI has trouble upgrading their armies (too scarce strategic resources being the other). I don't know how anyone could tell me they're OK with this with a straight face.
I appreciate what they were trying to do, but there's just not enough units for so many sub-roles. At the very least they should allow most units to optionally upgrade to the other sub-role (i.e. Warrior -> Pikeman, Knight -> Cavalry), but that's not enough to fix just how wrong the current execution feels.