I think basically the same thing ought to be done with ALL the upgrades in the game. As in, the Blacksmith have upgrades for Piercing Armor for Infantry specifically, and each upgrade would be like 0.5 - 1.0% increases at a time, and would either take a lot of resources and time to upgrade (increasing as the overall level goes up) OR cost Crowns to finish instantly - or at least quicker/at a lower resource cost.
Like Russell said, it would reward those players who actually play the game for a long time and love the idea of day by day, hour by hour, improving their civilization bit by bit. The instant, cheap, and single upgrades of the current system are terribly boring and don't help give a reason to spend crowns, or build more research buildings, or even give the satisfaction of achievement at all.
If the upgrades were similar to Russell's suggestions and as upgrades in the civilization's research buildings, players would get that sense of accomplishment, it would force them to consider how to spend their time/resources, differentiate players from one another, give another reason to buy/spend Crowns (i.e. more money for you developers), and overall make the game way more interesting.
In short, when I can get all the upgrades ever before I even have a palisade wall around my dinky village, something is wrong.