does it really make great advantage the migration java 8 project to java 9 modularity taking into account the cost benefit?
which size of project would be worth applying modularity?
For migration, I wouldn't just look at sheer size. Migrating a 'big ball of mud' vs. migrating an already nicely decomposed system is a completely different story. In general, however, using the module system is beneficial in medium to large applications. That's where the benefits of strong encapsulation and explicit dependencies come out best.