As long as they're already writing new code in Kotlin, translating the existing code makes a ton of sense, if they can do it cost effectively (which is sounds l

As long as they're already writing new code in Kotlin, translating the existing code makes a ton of sense, if they can do it cost effectively (which is sounds like they did). One of the huge problems with a language migration is that you're left with old and new and all the parallel tooling, context switching, and impedance mismatches. There's often a claim that the existing code will be migrated eventually, but IME that doesn't actually happen with large code bases. Then if there's ever a third migration, you're left with three languages in play. It's much better to aim for 100% migration, as close to 100% automated as possible. Then when you're done, you're really done. Maintenance, training, and the next migration will be easier.