The big cloud vendors have partners who specialize in such work. Random one from the first page of Google results: https://aws.amazon.com/blogs/apn/migrating-a-mainframe-to-aw...
part of this is the 'partnership' arrangement.
Global 100 company gets IT from other global 100 company
vs
Global 100 company gets IT from small mainframe support shop
there's a question of shareholder liability, being able to adequately sue them for M's of $, expectation they will be around in 20 years, etc.
Who are the vendors that provide a migration path from mainframe to a set of commodity hardware running an emulator, and subsequently provide continual maintenance and support?