The point is that whatever secure messenger you use, it must plausibly be secure. Email cannot plausibly be made secure. Whatever other benefits you might get from using it --- federation, open source, UX improvements, universality --- come at the cost of grave security flaws.
Most people who use encrypted email are doing so in part because it does not matter if any of their messages are decrypted. They simply aren't interesting or valuable. But in endorsing a secure messenger of any sort, you're influencing the decisions of people whose messages are extremely sensitive, even life-or-death sensitive. For those people, federation or cross-platform support can't trump security, and as practitioners we are obligated to be clear about that.