It's normal, many RTS don't have progression. Sometimes people don't feel the technological improvements.
Age of Empires 2 isn't a good example. In AoE, there are eras, while in 0AD, there aren't any; you don't go from the Bronze Age to the Iron Age.
That's fine for ages, in 0AD there are no ages, the equipment of a centurion is not much improved compared to that of a princeps.
I recommend you see it more like Total War series logic.
Again, this isn't about turning 0AD into Age of Empires, Delenda Est did a better job with scouts.
Putting centurions instead of a princeps makes no sense.
It's like evolving from a corporal to a lieutenant.
In Age of Empires, it goes from upgrading a 18th century militia to a 21st century marine.That's the logic of AoE. If you want real changes, it's better to differentiate the equipment.