While I agree with your conclusion I don’t think reliability is the best reason to switch. Signal is probably less available than FB services and many popular Matrix servers have semi-regular downtime as well.
Although in favour of this argument:
Setting up a “backup” service for when FB is down is a nice way to gently ease people onto something better.
Matrix or XMPP does allow you to be in control of the availability, (although to be honest you will probably struggle to better than FB Messenger or WhatsApp).
While I agree with your conclusion I don’t think reliability is the best reason to switch. Signal is probably less available than FB services and many popular Matrix servers have semi-regular downtime as well.
Although in favour of this argument: