That's not really true considering in about a week I know of a region that had lost 3k peasants limiting its production considerably for quite sometime. Also the "if you can loot all the regions why not take the capital, I know of one example where they couldn't take the the capital but could loot the rest the regions. Carelia's army could not defeat the Suville army looting it but Suville couldn't attack the capital because with the long time militia Suville's army would outmatched.
If it's a single region that lost 3k peasants, then it will recover quickly because all of your adjacent regions will be pouring in theirs, assuming that the local region lords know what they are doing.
If many of your regions are down 3k peasants, then yes, you're in trouble, but the enemy had to take a lot of time and effort to achieve that.
