There is no single right answer for modernizing Fortran. Here is what the realistic paths actually look like, and what matters most when choosing between them.
Fortran is rarely "modernized" in the enterprise sense at all. Its buyer is scientific and research computing, and the active investment (Fortran 2023, ongoing coarray and interoperability work) is aimed at keeping numerical performance competitive, not at replacing the language. Where Fortran codebases do get touched, it is usually to add C interoperability (iso_c_binding) so newer code can call into proven numerical routines, not to rewrite those routines from scratch.
Keep the code, modernize around it
Vendors taking this approach for Fortran: The Fortran Company.
Rewrite or convert to a new platform
Vendors taking this approach for Fortran: Astradian Technologies.