To me, there are two separate issues here.

The first, which has been discussed in multiple threads, relates to the laws on emotional support animals, services animals, etc. The second issue related to the responsibility of Delta, or any other company, to comply with regulations in a way that does not impact the experience of other customers.

So in the case of the OPs description, whether the dog should have legitimately allowed to be in the cabin is one thing. More importantly is the way in which the animal infringed on the legroom of the other pax in that row. If the dog interfered with these pax legroom, then, IMO, the needs of one pax were met at the expense of others. These pax may not have cared; or they may not have felt comfortable saying anything. I frequently see the same issue with pax of size. Almost all of us are uncomfortable when our neighbor's love handles encroach into our space; but few of us are willing to say anything about it. Most just sit there unhappy.

Companies should make sure that other pax are unaffected (within reason) by accommodations made for others. If this means that an airline needs to leave an open seat, so be it. There is too much of a "let's hope they do not complain" attitude on the part of companies. And because many people do not want to be labeled as inconsiderate, cruel, or mean-spirited (some of you will think I am all of these things, based on the opinion I am expressing), they do not complain, even though their experience has been diminished so that accommodations could be made for others. To me, this is unfair.