F-90, I just arbitrarily picked your post to reply to.
I haven't read all the posts(too many), so my apologies if I am presenting a hypothosis that has already been floated.
1. Selling both airlines reduces liability. Let Trans States and Pinnacle worry about liability.
2. It transfers a rather large training cost/footprint to someone else. If DAL hires a Compass Captain, they will have to train him, but it should be the same no matter whom they hire. However, Compass has to train two. First, a Captain upgrade to replace the one gone to DAL, and then a new hire to fill the F.O. position vacated by the newly upgraded Captain. If DAL really takes a large number of flow -up pilots, they have just dumped the lions share of training costs on someone else.
Obviously this is just my speculations and the true reasons may be much more complex, but mitigating the costs of the flow throughs may be part of it.