If they stick with Qt, maybe they could share some apps with Sailfish. With some coordination, it should be possible to get a common repository "app store" that would also benefit the KDE Plasma Mobile project, and perhaps even Ubuntu Phone if Ubuntu would decide to restart development. Of course hardware support is always the limiting factor, but at least you wouldn't need a limiting phone-specific app ecosystem if a lot of Qt-based software would work on a wide range of devices.