Heirloom Computing, I am CTO, have transpilers for cobol and pl/1. code transpilation is the easy part. Create a Grammer for the source language, generate the ast, transform the ast to the target language ast and generate the code.
Now make that generated code run and behave the same way as on the mainframe and give it all the subsystems that exist on the mainframe for transaction, file handling, security etc. This is the part that is difficult. Fortunately it's already solved and in production with our clients.
So transpilers do work and are in production but our biggest competitor is inertia.