First off, curriassers cost the same amount either way. So either way, you should be able to cost effectively beat them using the appropriet counter unit.
So what seems to be the problem is that everyone thinks they can't mass produce the counters as quickly as they are going to need. But look at it this way, if he has enough money to afford 60 curriassers + constant replacements, you should have enough money to afford multiple barracks (or stables) to pour out the cheap counters. If you can't, they you probably would have died anyways as his economy must be crushing yours.
And even if you can't manage to replace at the rate he can, what is the big deal. If he went pure curriassers, he just has a bunch of cavalry running through your base, not doing enough damage to your buildings to offshoot the amount of money he lost in the battle against your army. If he is going to make a solid dent in your base, he is going to need cannons. but for every 2 cannons, he loses 5 curriassers, and every time he loses 5 curriassers, the effect of the card seems less and less and less.
So what everyone is calling 60 curriassers, will probably look more like 40 curriassers, and 4 cannons (if he doesn't balance out his army) or 30 curriassers and 60 slots worth of other units(if he does want a fairly balanced one).
so what's the big deal you may ask? It's simple, what everyone is calling 20 extra curriassers, in any practical army, will accually be 10-14 extra curriassers, which is far less troublesome. That is practically an extra 30-42 population slots in an army (up to about 150 population army, the same as the dutch already have), which is easily overcomeable, esspecially since you already know what the core of the enemy's army is going to be.
Finally, with all the complaining and moaning from all the people here, don't you think that ES would test specifically to see if that card was over powered or not? I think, if anything, when the game comes out, the card will be too weak, not too strong.