The Crux of BDD is... Gherkin!
Tests for the Whole Team!
Single Source of Truth
A Focus on Business Value
Given..., When..., Then... Structure
Now hopefully you can see how we can have these very high level, easy to understand Gherkin tests while still allowing them to get deep into the nitty gritty parts of the code and truly test the codebase the way unit testing always has. I think most unit tests already follow the given, when, then pattern in some form. They don't explicitly use those words, and they are usually all thrown into a single method, but it shouldn't be too difficult to see how your unit tests can fit into this pattern.