Except it isn't an app :), which is the reason for the original inspiration behind the project. Postcards aren't a very popular thing anymore (hopefully that changes!), and I don't quite believe a dedicated app is justified when you can do it straight from SMS or Messenger.
Postcards aren't popular, you are right. I don't think they will be anymore. Kids my age are sending snaps, and even archaic image texts. I think we are are more interested in showing our friends "I'm here, look at what's behind me", then in sending some generic, impersonal postcard which doesn't prove anything.