My grandma uses WhatsApp Web and Facebook hasn't taken down the WhatsApp Web snap yet. Both are end to end encrypted and based on the same transport but one of them used copyright law to take down a redistribution of their application.
If "a messenger that works" is a narrow lens, then sure. WhatsApp is still as secure as it ever was and everyone is already on it.
Signal had one feature it did better than its competitors and that's allowing integration of SMS. That feature is now getting killed because of RCS issues. With all my contacts on similar chat apps, I don't see why I should keep Signal installed once they remove the SMS feature, let alone why I should convince my grandma to make the switch.
"But Facebook is evil and wants to control you and wants to suck your blood" yes and so do the companies that made our phones and mobile operating systems. What's the point of Signal's openness (well, "open", they did stop uploading the source code for a while when they were adding in their crypto scheme) if you still use it on a proprietary phone.
And no, Linux on neither mobile devices nor the desktop is grandma-ready.
Looks to me like Signal is the interior app here.