Are there available numbers to support this? Software engineering in the U.S. is well-compensated. $200/mo is a small amount to pay if it makes a big difference in productivity.
Which raises the question: If the productivity gains are realized by the employer, is the employer not paying this subscription?
Basically what they did is they had repeated codemods (changes to the entire codebase with automated tooling) that bit by bit moved PHP closer to Java. More and more static typing, generics, all the common Java ADTs (Vector, Map, Set, Pair, etc.), bytecode+JIT execution, etc.
Essentially instead of rewriting the codebase to a better language they just changed the language itself. Which makes sense since PHP is a much smaller codebase than the Meta backend.