defer is still something you have to consciously put in every time so it destroys the value semantics that C++ has, which is the important part. You don't have to "just write defer after a string", you can just use a string.
The 'not a problem for me' is what people would say about manual memory in C too. Defer is better but it isn't as good as what is already in use.
If you want to auto-generate destructors, zig has really good comptime features that can let you do that.