BK doesn't just "write off the debt". The debt has to be resolved. Its not a "you get to keep all the assets for free". BK is "reorganization of debt". Spirit already did this. If they file BK again the creditors (the people that debt are owed to) would just ask the court to sell all the assets to pay them off or in the case of leased planes, will just repossess them.

There will be no merger because F9 doesn't want to deal with the $3B+ of debt that would now be on their balance sheet.