That has a much higher barrier of entry, though. If I work on a project written in Blub, I won't be able to do any metaprogramming unless I can get everyone els

That has a much higher barrier of entry, though. If I work on a project written in Blub, I won't be able to do any metaprogramming unless I can get everyone else to adopt my Blub++ compiler. If the software is written in Lisp, all I have to do is get it through a code review. In effect, it's not really scaling; it's being limited to the small portion of developers who work on compilers.