I wouldn't go so far as to say you are wrong, but just because major airlines CAN terminate their regional contracts if that regional files for bankruptcy, it doesn't mean they will. Bankruptcy offers a chance to restructure debt repayments and cost structures, among other things. If Mesa, or whichever regional, filed bankruptcy, it may benefit the mainline carrier to keep the regional relationship active for a while. Simply put, the major partner may want to see IF restructuring is possible, and if that restructuring may lead to lower costs. If our hypothetical bankrupt regional were able to cut costs by 20% or so, it may be able to keep some or all mainline flying by offering to fly for a lower reimbursement rate.
If Mesa went into Chapter 11, they could crunch some numbers and make new proposals to their contract partners. As long as some major is interested in Mesa's new offering, Mesa can proceed to try to make that proposal a reality by seeing if "future" surplus aircraft can be returned or sold, if needed leases can be renegotiated, and if other debts can be reduced. If Mesa could get all those ducks in a row, it can offer up a business plan to the judge that shows how it intends to meet its debt obligations, and how it plans to run as a healthy business with the new fleet and contracts. Say Delta pulls out of its contracts, but US Airways and United stick around to see what bargain could be had. If Mesa can make the new numbers work, and all the creditors are happy with the new business plan, Mesa could exit bankruptcy leaner but alive. Of course, someone would have to sponsor their exit. You never know who could step in. GE may want to keep those CRJ engines flying, or maybe Bombardier doesn't want to get zero return on those planes if Mesa shut its doors. Lots of possibilities.
In reality though, with everyone looking to reduce regional lift, I think any regional that filed Chapter 11 would have a hard time exiting unless another airline bought them. Look at Frontier. Good company, profitable while in bankruptcy, positive future, but only other airlines would give them a serious look. And any regional is far less attractive to purchase than Frontier was. As for Mesa, I think if one partner decided to end its relations with Mesa, that Mesa could not sustain itself anymore. The margins are too thin.