High complexity low quality legacy Java code. How do I motivate you to work on it?

Title gives it away. Managing a massive Swing Java GUI software. Many developers have come and gone. Quality is mixed. Testing is minimal. The software started in a different domain and mutated into 100K lines that run our business. Many attempts at rewriting failed. The domain is very deep (related to CAD). Is this a corner case or more common than we like admit? What would it take to get good developers to come and stay? Having a hell of a time hiring and keeping engineers.