A lot of COBOL software is running on IBM mainframes under z/OS. In that case, it isn't just COBOL, it is all the middleware and OS components the COBOL interacts with – CICS, IMS, DB2, VSAM, JCL, DFSORT, etc, etc, etc. Duplicating all of that functionality is an infeasible task. Duplicating a big enough subset that you can successfully port a real world mainframe app is achievable, and a number of commercial offerings exist which do that – e.g. Oracle Tuxedo Application Runtimes for CICS/IMS/Batch, Tmaxsoft OpenFrame, UniKix, etc. However, building such a solution is a huge amount of work requiring a rare skillset, and I doubt the open source community has the ability/resources/motivation to replicate that work.