> So what does unique value proposition does GHC have left? Possibly the GHC runtime system, but it's not as sexy to pitch in a blog post like this.

The point is that programming in a pure language with typed side effects and immutable data dramatically reduces the size of the state space that must be reasoned about. This makes programming significantly easier (especially over the long term).

Of the languages that support this programming style Haskell remains the one with the largest library ecosystem, most comprehensive documentation, and most optimised compiler. I love lean and use it professionally, but it is nowhere near the usability of Haskell when it comes to being a production ready general purpose language.