Does it really matter so much? In some apps I even get the option to open either Apple Maps or Google Maps and otherwise you can just use what you want. I have both on my iPhone along with (former Nokia) HERE Maps which has excellent offline navigation. Google Maps has street view and superior information on point of interest. Apple Maps looks better, feels somehow faster, does everything like it's supposed to nowadays and even provides with information on public transport now in my country.
I wouldn't say any of these third party apps should be "the default" because each has their own strong points. You can't seriously think Apple would integrate any third party app as well as their own apps. That's not even quite possible.