What pisses me off about many such endeavors is, that these companies always want big-bang solutions, which are excessively hard to plan out due to the complexity of these systems, so it's hard to put a financial number on the project and they typically end up with hundreds of people involved during "planning" just to be sacked before any meaningful progress could be made.

Instead they could simply take the engineers they need for maintenance anyway, and give them the freedom to rework the system in the time they are assigned to the project. Those systems are - in my opinion - basically microservice systems. Thousands of more or less small modules inter-connected by JCL scripts and batch processes. So instead of doing it big bang, you could tackle module by module. The module doesn't care in what language the other side is written in, as long as it still is able to work with the same datastructure(s).

Pick a module, understand it, write tests if they are missing, and then rewrite it.

After some years of doing that, all modules will be in a modern language (Java, Go, Rust, whatever) and you will have test coverage and hopefully even documentation. Then you can start refactoring the architecture.

But I guess that would be too easy and not enterprisy enough.