I agree with most of your comment, but I don’t think this part generalizes. This isn’t feasible for most companies unless you’re operating at Facebook scale.
Afaik, the team that did Hack/HHVM at Facebook is ~5 people. I don't think you need scale for this (the rewrite of the code itself is not a scale thing, the codebase is usually linear-ish in the number of engineers).
My point is: instead of doing a rewrite, be inventive and avoid it, and this is a great example.
Also, not every company could hire the 5 people needed to do their own version of a hack/hhvm project. But at some point it makes sense to find those people instead of rewriting in an unrelated language
Facebook has the scale to invest in creating tooling, IDE extensions, core libraries, documentation, training, test frameworks, bindings, etc for a new language that they create. They also have the organizational scale and career development to make it worthwhile for an engineer to learn their proprietary language.
This doesn’t apply to most other companies.