DAL management most likely knows what they are doing, with both freedom and pinnacle. I'm pretty certain that the statistics they are using are accurate, although as someone pointed out they may be using weak contract language to punish 9E for performance issues outside of their control. This is not nice, or particulary "fair" but it is legal.
It is entirely possible that DAL has thrown out the poor stats which could be blamed on them, and are basing their decision solely on performance which is directly attributable to mesa/pinnacle. My gut feeling is that mesa would have screwed themselves regardless of what DAL did, or did not, do.
Also, DAL might be basing the decision on numbers from several months ago, or longer. The contract probably says that if you fail to perform, you can get fired. There is probably nothing that says they can't wait for an extended period and THEN fire you for something that happened last year...effectively, if you screw up one single time then you are an "at will" subcontractor for the remainder of that contract. DAL can always pull that out of their hip pocket at any time...in fact, they LIKE it that way. The fact that you have improved since the original failure means nothing.
DAL was probably in a position to strongarm pinnacle into signing a weak contract, so they might not have had a choice (or they might just be stupid).
More than most other majors, DAL seems to have their poop in one sock. If they canx MY contract, I would be concerned. Yes, anybody is at risk right now.
The good news for 9E is that those 900's are more in demand than most RJ's.