Whether it’s mainstream is debatable, but I think Elixir is a really successful example of extensible language, and indeed it managed to do this without s-expressions. Various libraries extend the language for HTTP routing, parsing, DB queries, property testing, static analysis, etc. It makes possible a lot of experimentation by the wider community, and not just the group of core language developers.

The creator has repeated this philosophy a few times:

> There is very little reason for an Elixir v2.0 with breaking changes. The language was designed to be extensible and if we need to do major changes to the language to improve the language itself, then we failed at the foundation.

https://elixirforum.com/t/what-would-you-like-to-see-in-elix...

> A big language does not only fragment the community and makes it harder to learn but it is also harder to maintain. It is also why the language was designed to be extensible: so the community could build what is necessary without a push to make everything part of the language.

https://elixirforum.com/t/questions-about-property-testing-s...

> We also understand there is a limited amount of features we can add to the language before making the journey too long or the language too big. Adding something now means not including something else later. As an exercise, let’s see a counter example of when we didn’t add something to the language: GenStage. [...]

https://elixirforum.com/t/questions-about-property-testing-s...