There is no single right answer for modernizing PL/I. Here is what the realistic paths actually look like, and what matters most when choosing between them.
PL/I's installed base skews toward European banking and airline systems rather than the US commercial mainframe base most modernization vendors are built around, so fewer vendors have deep PL/I-specific experience compared to COBOL. Its ON-condition error-handling model also does not map cleanly onto exception handling in Java or C#, so a straight syntactic conversion tends to lose the original error-handling intent unless a human reviews it condition by condition.
Keep the code, modernize around it
Vendors taking this approach for PL/I: Rocket Software, Micro Focus (OpenText), OpenLegacy.
Rewrite or convert to a new platform
Vendors taking this approach for PL/I: Astadia, TSRI, Accenture, Capgemini, Ispirer.
Keep it running, don't change it yet
Vendors taking this approach for PL/I: IBM.
Fill the workforce gap without a migration project
Vendors taking this approach for PL/I: TCS / Infosys.