Why Manual Code Review
Java is very mature.
Users struggle with mixed patterns from Java 6, 7, 8, 9.
Testing does not help understand what is in a code base. The code is a mix of unconsistent things that will go through every testing stage.
Manual review is the only way to keep production in sync with tecnology objectives.
Churn of developers and testers at your department is huge. Nobody really knows what is in your software product.
You must rely on source code control, but this is a low level tool. It cannot guarantee the consistence of your business objectives with the software modules you have.
Periodical Manual review is the only way to keep production in sync with tecnology objectives.