If it was a wave based survival mode which increases in difficulty, it could be that you can only receive loot for a wave once per week, meaning you have a possible infinite amount of loot, but if you wipe or stop, you have to start fresh and won't receive loot for any hordes you have already defeated. With the rewards having an increased chance of better loot the further you get. This would be perfect in my eyes, but only on the condition they are very good at limiting the cheese, otherwise one invincible spot from which you can kill everything becomes game breaking.