This is an important point. Sometimes when you need to do a major rearchitecture of a system it can help to choose a language that is more appropriate to that architecture. The Elixir/Erlang ecosystem has a better story for Actor Model development so it makes sense to choose them for the new architecture. It depends on the team and the specifics of the new architecture because the devil is always in the details. It may be that the new language isn't enough of a win to justify the switch and sometimes the win from the new architecture is big enough that sticking with the current language makes sense.

But a knee jerk response of: This is mostly just good because they rewrote/rearchitected it, ignores the benefits of using a language or technology that fits the new architecture better.