Have seen similar transitions in different companies, and while I can't say anything about Heroku, it is often not the target technology or architecture that matters.
For instance I've seen PHP -> nodejs transition while moving to micrservices, and while the ideas made sense on paper:
- It didn't come from the engineers at large. Most weren't phased by the prospect and the main engine for the change was the top architects and the engineering management.
- target architecture and language were very popular, and easy to hire. Incidentally salaries would be cheaper for same level of experience engineers.
Predictidbly a ton of the existing engineers left and new blood came in, and it was mostly according to plan from what I gathered. Some of the "old folks" stayed at a premium once they were the only ones left with enough knowledge, but they got relagated in side "experts" roles and the product as a whole saw a big change in philosophy (I think mentally what was seen as the "core" also became "legacy" in everyone's mind as engineers moved away)