That’s exactly your one mistake!

That’s your second.

The reason the purge is expensive is your mistake, no others.

Give time to train better thralls and gear them strong enough to endure.
But, never forget… This purge is your option, no others. The game does not force you to do it.
I can understand wishes for better loot, even crafters in boxes and i am all down with it, but presenting 1500 obsidian arrows and a Darfari soldier like something big, i am sorry it’s just wrong. Darfari lvl up in a heartbeat and obsidian arrows are really cheap, so technically your loss is zero.