We migrated the core banking from a small private bank, and that was 12 million COBOL lines. It actually seems like a "small" system. We used the same approach (java translation). Running the cobol is actually the easy part. The hard part is when you start throwing in managing files the mainframe way, the database (needs to quack like DB2), the transaction monitor (CICS), presentation layer (3270 like), scheduler, etc. And there is plenty of other technologies usually deployed in such shops (IMS, MQ, etc). And you need to support it all...