But the options aren’t “manual” and “ai”. There is a third option of using classical static analysis and compiler techniques that have been around for decades to do transpilation and large scale refactors.
There are entire companies already build around this.
There's also a fourth option: do nothing. Leave the existing services in the languages that they're written in rather than translating them into other languages and laboriously debugging the resulting code. If these services communicate through well-defined APIs, any new services, which can be built in new languages, can communicate with the legacy services.
It may be more cost effective to keep around some developers who know the old code base and the language it's written in.
There are entire companies already build around this.