There can be substantial differences among what's technically feasible and what's economically feasible and what's politically feasible.
An existing environment is usually a large pile of twisty dependencies on operating system calls, on database or file formats that aren't entirely portable, on language extensions that don't exist elsewhere, on objects built from source code that's been lost, on dependencies on products from vendors that no longer exist, and it's all often inextricably linked with front-ends that would have to be rewritten, data bridges that would have to be rewritten, and users that need to be retrained.
In some cases, this effort can involve re-architecting substantial parts of a manufacturing facility that's involving six or ten buildings and that are each most of a kilometer long. And with many industrial-scale environments, change is not going to happen. You (might) see a new environment implemented when the entire production line is nuked and paved at the end of its useful lifetime, and replaced with a newly-deployed production line. In other cases, you'll probably see the existing system patched and re-used.
For many of these cases, just ramping up a load test would be a substantial effort.
Some hunks of the environment will have test cases and specs and documentation, and some hunks probably won't.
For many of these installations, substantial downtime has SEC-level visibility, or can involve exposures to various legal entanglements, or can involve re-establishing politically- or legall-mandated external certifications, or can cost thousands or millions of dollars per unit time off-line, or requires the production line to be cleaned with a jack-hammer or entirely replaced, or other issues.
If you can afford an incremental port or a big-bang port, well, what's that cost in comparison with the incremental costs of keeping the existing environment going?
C-level folks just don't blindly sign the really big bills involved in keeping these environments going, and most competing vendors seeking to acquire least part of these mainframe-scale budgets.