It all depends on how Jolla implements the Android support in the future releases. If they stick to AlienDalvik, we will be stuck on 4.4 forever since Dalvik was replaced by ART starting with Android 5. Furthermore, Myriad (the company from which Jolla licensed AlienDalvik) seems to have abandoned AlienDalvik, which is not much of a surprise since it's pretty old by now and support for Android 5+ probably needs a total redevelopment.

The only alternative for newer Android support seems to be Anbox, which look promising. But I'm not sure it's already stable enough to be used on productive devices.
We'll have to wait for more announcements from Jolla.