Wouldn't it make more sense to do a massive refactoring given the situation is as you described? And, if needed, rewrite to Kotlin later. This would just cause the same problems to be reproduced in a different language.
A rewrite, too, would take less than years, would have a better end result. Honestly the FB application is not exactly rocket science, and the massive amount of code they have points more on the direction of broken internal process and a total lack of simplicity in the design and choices.
