I think it’s a good idea from a security standpoint to have a UX space in which everyone can be confident that everyone’s stuff is encrypted; with a very distinct and (yes) inconvenient barrier — in this case, a different app — between encrypted and unencrypted spaces.

Everyone is using lots of different messaging systems: SMS/MMS; specific systems like Signal, Telegram, or WhatsApp; email; maybe Facebook Messenger; etc. It’s really important for some users’ actual lives that it be totally clear when you’re crossing from a secure space to an insecure space. Having the insecure space not be in the same app is one way to accomplish that.