I agree it doesn't generalize, but I don't agree that it's because of "scale". It doesn't generalize because not every company should write its own language (and can't hire good enough engineers for this). And, fortunately, most "legacy" codebases aren't in "bad" languages, so there's no need for this.

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.