It's all cost related, the WO's will become more common because the CPA cost will go up because there are fewer companies bidding and they can take advantage of that faster than a pilot group. Compass was the only E-jet operator that wasent already growing and therefore could actually staff the planes. As more contracts come up the bids will be higher shifting the flying to WO carriers.
If a regional can make 15% return with better CPA's the mainline partner as might as well own them because that's their margins.
Same reason Delta bought Comair, at that time there weren't enough operators for jets as fast as they wanted them in service. So regionals were turning higher margins, then more operators entered and drove them down. Finally the pendulum is swinging back.