Saying that it's 2015 is a bit misleading. In JS's case, it's mostly a question of backwards compatibility and interactions with those backwards-compatible features.
The designers of let are largely a bunch of Schemers; they knew what they were doing.
I see why my post was confusing. Let me clarify: I was referring to languages like Coffeescript, not let in JS. Though JS got scoping wrong 20 years after it was a solved problem...
The designers of let are largely a bunch of Schemers; they knew what they were doing.