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.

When we need to move data between the secure space and the insecure space, we can do that through copy-and-paste, or even screenshots. It is inconvenient, but that's because it's explicit and intentional, which also means you can't move data from one to the other by accident. That's good.

As a privacy hobbyist, I want to notice what works for the people whose lives depend on privacy: the journalists, activists, sex workers, LSD dealers, etc. I don't have their risks, but I want to contribute to a world where they can be safe.

However, there are definitely lots of different needs and comfort levels. What's a sweet spot for me might be an uncanny valley for you.