You're trying to rationalize why, and I think you're successful.
Not sure who is next on the Delta chopping block, and I don't really buy into the idea that they want to replace mega cheap -900's with expensive diesel 9's. Quite simply, they're cleaning up shop for the merger, with whatever rational they want or need.
Our company wasn't successful in the contract that your company got (and we had/have the planes, pilots, and mx ready to go). I believe that we were underbid to the point that Jerry didn't think we'd make any money. Also, I know we lost the LAX Delta flying to XJT, in part, because they wouldn't pay to have two spare aircraft. I guess your company got zero.
Have you considered that this might come from your own company as a way to get out from under a looser of a contract ?
I'm confident that in a worse case scenario, those -900's won't just sit around and drag the company down. They are a hot commodity, and will be easy to unload at a premium.