I don't think that your service based model is more extensible. Yes, I could build a component based version on top of the service based version. But I can also build a service based version on top of the component based version.
In fact from a peek at the source, the NgbModal class is built on a NgbModalWindow class which acts much like react-bootstrap's modal. So, ng-bootstrap builds the service based version on a component based version. If either library can be accused of bloat it ng-bootstrap for including the serviced based version when I might just want to use the NgbModalWindow directly. (Its not in the documentation, so I wouldn't actually use NgbModalWidnow directly in practice, even if I'd want to.)
Now, I've drunk the react cool-aid, so take my opionion with salt. But in my assesment while modals can be somewhat awkward in the declarative style, they aren't awkward enough to make me break my rule of avoiding imperative apis.
