Sure, but they have no reason to choose mer over the PureOS distribution that they already maintain
Why don’t you build a free UI ontop of Mer (Sailfish OS)? Or resurrect Firefox OS? Or insert-name-here? Because we want to promote a pure and unified stack, not have a separate mobile OS with proprietary bits or a completely different middleware stack. We want to support the community efforts of GNOME (as well as KDE) and allow for any GNU+Linux to work out-of-the-box providing mainline improvements that work not just on mobile but across the device spectrum. The Librem 5 is a new approach to use a regular Linux system and adopt it to mobile use-cases instead of creating a completely new system. We do not create a walled garden, instead we tear down these walls, creating an open utopia. A fully standards-based freedom-oriented system, based on Debian and many other upstream projects, has never been done before–we will be the first to seriously attempt this.