You should take a look at my company. Heirloom Computing. Heirloom.cc We have migrated many mainframe application and millions of lines of cobol and pl1 into Java and deployed it into production on prem and into the cloud.
It's a demo but the code that is transformed is trivial in the extreme. The whole approach of selecting small pieces is just not scalable. One small project i worked in was 22 million lines of cobol code, spread over 4000+ programs imagine having to select bits of code out of that.
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.
Heirloom computing where I am cto does this using transpilers with 100% automated transpilation. Using LLMs for an entirely deterministic domain borders on the insane. This is just marketing bs but we get asked about it and what our plan is to counter it all the time. Explaining that using Gen-ai and LLMs for what is a well understood compiler/transpiler problem that is already solved just seems to be too difficult for some people to understand.
Or any of the companies that can run mainframe COBOL and provide all the ecosystem. heirloom computing is of course the best (i am cto) but Micro Focus, lzlabs etc all enable you to run off mainframe with a full ecosystem.
Do not know about our competitors but you can run our product for free on a raspberry pi if you just want to play around.