I don't think a well designed capability system would be so onerous as to be unworkable, people use other complex features in Rust and C++ and Haskell just fine

I don't think a well designed capability system would be so onerous as to be unworkable, people use other complex features in Rust and C++ and Haskell just fine. It's just that being capability-safe by itself is not enough to carry a niche language. Eventually these ideas will find their way into more mainstream languages, one way or another.