Because the more I think about this the more it works on all levels and for all reasons!

Cas is a SOLDIER who FELL. And he fell BECAUSE OF DEAN.

Dean is the soldier who went on without his brother-in-arms, like Sam went on without him, like Cas went on without Dean.

When Sam first went on to live without Dean he worked with a supernatural creature, when Cas went on without Dean he did the same, now Dean is doing the same with Benny (well, he’s not working with Benny right now, but he and Benny certainly have a relationship of some kind at least.)

Sam and Cas both in their own ways betrayed Dean when they left him behind to work or live on their own, even though they had reason to do what they did. Now Dean is the one who has betrayed Cas, even though his choice was perhaps a fair one.

It’s just an extension of Dean following the path the others have already walked, which makes it PERFECT. YES.