Well, there are two party false assumptions in this.
First, most of the people in this discussion are vastly underestimating the negative impact Apple Maps have worldwide. Parts of the USA may be ok, some maybe even better than Google Maps, but other countries are a disaster. People all around the world who used to depend on Google Maps on their iPhones will have a very tough time ahead of them.
Second, iOS does not have the same market presence worldwide as it has in the US. Therefore, a lot of US Apple fans assume, based on what they see in the US, that Google should strive to "convert" iOS users to Android. But worldwide, the situation is much different, as there are millions (billions) of users still using featurephones. And those are the ones that both Apple and Google are targeting the most. And when those millions of people will choose their first smartphone, they want one which has all the functionality they expect, including maps that cover their countries. And guess which one they will choose when they compare iOS and Android? The amount of users Google can gain this way (by maybe not releasing a iOS app) is much much bigger than what they can gain from current iOS users.