Let's say unit A produces half the gold 5 workers would get from a mine per minute and unit A costs 5 food
You would need 2 unit As to replace the income from 1 gold mine, and it would take 10 food
Normally unless you are playing as ud you would use 10 food for workers (5 food for 5 workers mining and 5 food for 5 lumber gathering)
If you have the tradition 100 food limit this allows 90 food for your armies
Lets say your mine is out, you have to get 2 unit As to replace the income, so now you have to waste 15 food for the same amount of resources that you previously wasted 10 food on, although now you don't get taxed you have 5 less food to spend on your armies, that's minus 2 basic infantry soldiers, 1 tier 3 tank, or a flying unit.
Now let's say that unit A cost 7 or 8 food, with 7 food now you have 81 food to spend on your armies, with 8 food you now have 79 food to spend on your armies; although the income is no longer taxed (although you could set up triggers to tax it if you like) you now have less units to work with at any given time. This provides a balance if you do it right.
The player can build more unit As if he likes, but that food is taken from his other forces, this is the key to the balance.