So is let in some languages. In Rust, you have to constantly opt out from immutability with let mut, which makes writing more procedural code feel like you're fighting with the compiler, and otherwise I don't really see the rationale behind full functional coding. I only had a bug caused only once by unwanted mutation, the hardest part fixing it was to learn the proper use of my debugger tool.

oh for sure, but I think that's the rarer case for language implementions. Having a consistent structure with alternative keywords in static positions is just easier to develop an AST for. Personally my favorite language doesn't even allow for const values (except by convention) so it's really just a matter of preference