You don't need particularly tight integration with the operating system to make a good maps app; you'd want access to CarPlay, which Google Maps has, and you'd

You don't need particularly tight integration with the operating system to make a good maps app; you'd want access to CarPlay, which Google Maps has, and you'd probably want a good sharing extension, which I don't think Google Maps has but could if Google cared about making better iOS apps than they seem to. Google is rather notorious for being very, very slow to adopt new iOS features in their apps, and it's definitely not because Apple is making it difficult for them to do so. The history here is also at least worth a passing mention -- the iOS Maps app originally used Google mapping services. The reason Apple built their own back end is because Google refused to give them access to vector-based maps and turn-by-turn navigation unless Apple shared more user data with Google than they were willing to provide.