That's a fair point and a real tension in the design. Offline progress is capped the game won't simulate more than a few minutes of catch-up time when you reload, so threat can't spiral to 100% overnight. But you're right that even a moderate threat bump while you're away feels unfair when you had no chance to respond to it.
One thing that helps: turrets keep defending raids even while you're offline, so if you're set up defensively before you close the game you're in a reasonable position when you come back. That said, I hear the feedback, offline time accruing threat with no way to prepare for it is worth revisiting.
Appreciate you flagging it.