A counterpoint: Babel's extensibility doesn't matter in practice at all, other than helping the Babel team organize their code.

Pretty much every new ES6 feature required parser and babel-core changes just to be able to be used.

Example: a lot of changes that only worked in Babel 7 (that was on Beta for months) were not possible in Babel 6, and so on for previous versions. A plugin was not enough: you also needed parser/core changes.

Other than for novel non-standard features (like code substitution), plugins are not exactly that powerful, and even things like that are frowned upon in most environments, as 99.9% of people just want ES6 features.

> Preparing data for external use always takes extra effort.

How much effort are you really talking about for a static site though? Can you be specific?

Most sites I've worked on are processing a modest number of Markdown + JSON files, where sometimes these are pulled in from an external URL. Why does any of this require anything particularly complicated or anything that could justify a big performance hit?

> You can build an efficient self-contained tool in JavaScript too.

Does one exist for static site generators though?