The sunk cost fallacy suggests that sometimes is better for you if you should just accept existing losses, accept you've already sunk those costs and won't get them back, and move on. I don't know what their contract looked like with the agency, but an 80% of a logo design sounds like a perfect deliverable that you can safely fire the existing team and take the 80% deliverable to a new designer, not start from scratch, and ask them to do a final polish step. I would have cut losses there, but of course it is much easier to armchair quarterback from hindsight and different perspectives than if you are in the middle of it fighting that gut feeling that you've already invested so much and can't "afford" to cut losses.