I definitely agree with you that the decision shouldn't be a dick waving contest.I don't want to be pedantic about Dortmund but it's not been low attendance in Europe, domestically they are allowed 80,000 in there.
I just don't have a problem with empty seats. Lyon were a good 12k below capacity and the atmosphere there was great when we played them. People bang on about Dortmund, but they were regularly 10-15k below capacity in the Champions League this season. Valencia and Galatasary are often below capacity. Napoli and Marseille are regularly 30k+ below capacity.
You can't call any of them bad atmospheres. With a well-designed stadium and smart ticketing, a few thousand empty seats shouldn't make a difference to atmosphere.
When in Europe their capacity is 65,000 and their average attendance in Europe this year has been around 63,000.