That is a mechanic everyone had to learn the hard way. You fail, you get experience, you improve. I understand this may cause different players to get different reactions yet there are ways to avoid it in the first place. And I am not even talking about "placing a couple low tier units in the garrison" I refer to going to the map settings and deactivating/lowering Roaming Unit Spawns.
The good thing of AoW games, and it seems AoW4 will be no exception, is that you can tailor your game experience however you wish to do so.