Hate to break it to you, but anyone that is thinking that Mesa's downfall would lead to anything other than a reduction in flying at US Airways need their head examined.
Quite simply, the E-190's and Republic can and will replace the CRJ-900's, and the 50 seat flying that Mesa operates for US Airways will simply go away.
The most likely area of "growth" for anyone from Mesa going under, might be with the Dash 8's, but even then there is no guarantee that it would come to Piedmont (instead of a cheaper vendor)...
No, Delta's getting rid of Mesa because they don't need them at all. US Airways needs Mesa to a smaller degree, but not to the point at which it will not take advantage of the golden opportunity to reduce it's RJ fleet.
The only chance Mesa has is that in BK protection, US Airways cannot shed them. IF Mesa is able to find financing for a restructuring (good luck with that in today's credit market), you might see a Mesa version of what happened at US Airways in the past, where Mesa itself sheds it's 50 seaters, obtains stiff concessions from it's labor, and retools itself to fly only the 900 (and possibly the DHC8) at even cheaper rates than they offer now.
Doubt it.