Well, I disagree with you here.
In Heroes 1, there are no upgrades!
In Heroes 2, there are upgrades, but not for all units...
In Heroes 3, there are upgrades for all creatures in the castle.
What we see is that there is development happening...
Furthermore, the fact that the hero has become not only a commander but also a fighter is also a development. The same applies to the hero's characteristics.
You can't stand still... you'll get covered in moss... :D
The world is changing, and we must change too...

Sincerely.