Really? I haven't found it an issue to get friends or family to sign up for Element. In fact it's even better because you don't need to use your mobile number for that.
Haven't used Element but that sure doesn't sound better. Signal works as a drop-in replacement for the default SMS app on Android, I'm not gonna tell my mom to stop using my phone number and message `tfehring` from Element instead (if it's even that simple), especially since she'd still need an SMS app for everyone else.
I've found this specific issue doesn't matter for 30% of mobile users, because they're using the phone purely in a reactive state (usually to popups, or the notification area!).