Yes, sure, but why not simply point out that the communication between Signal and Whatsapp, for example, is not sufficiently encrypted? If someone doesn’t use Signal or Theema, you can only communicate with this person anyway if you use the corresponding app. That’s not any more secure. I just think that Signal & Co. could gain a lot of users if they also allowed (insecure) communication with other messengers. Encryption between users who both use Signal, for example, is not affected by this.