This is a weird way to think about it. You are basically saying a company should not launch something exclusive to their platform or ecosystem, but rather should consider launch a generic product compatible with everything out there. Why would they ? How will they stand different if everyone does that ?

Exclusivity is a basic part of business model. Look at PS4 with exclusive titles. Hell, look at your local store with exclusive products only available in their stores.

I would have agreed with you if Apple had done this for a basic feature like calling. But this sure is a privileged feature and there is nothing wrong in making it exclusive to iPhone (but they haven't you see).