Earlier quoted context omitted.
So how about don't be stupid about it and instead make three engineers devote 1/3 of their time to this? That way it's a broadening rather than a redirection of their career, they can help each other, the on call duties are shared, and the business isn't dependant on a single person.
Dedicating a third of my time to a language I had no previous intention of learning is going to be a hard sell, even if you label it as "broadening." If I'm a python developer, I'm now spending less time on my core competency, giving other python developers an edge. Seems like you're actually narrowing my career, unless I wish to apply for jobs seeking mediocre Erlang programmers...
The workers are settled and and don't want change. And there's nothing wrong with clocking in and out and not giving a shit about your "career". But there is something wrong with the managers falling for this.