I can understand where you're coming from with the split between API and impl. It's certainly something I'm keeping on my radar. Personally I care more about readability than writability so it's not a big issue for me, but I can understand that others would land differently on this point.
One reason that I'm holding out on this (for now) is that I'm actually excited about how this one decision makes it quite easy to express some quite complex patterns using "Build Time Metaprogramming": https://docs.clarolang.com/metaprogramming/code_reuse/reusin....
The whole section is really more of an exploration than it is any sort of strong statement that this is what all Claro code should look like. But it's all fundamentally enabled by this "physical" separation between API and impl.
I don't like the separation of of interface and implementation at the file level. I feel like having to type the same thing twice header-file-style really slows down development iteration speed.