While it is true that one cannot set Google Maps to be the global "default" when tapping on addresses in non-Google iOS apps or when using Siri, I found it is still possible to launch Google Maps from within Apple Maps in iOS 9, though it appears Apple has changed things a bit from how it presented access/pass-through to "other" routing apps in iOS 8.
Once the address has been tapped and the route is brought up in Apple Maps, tap the car/travel time image near the dropped pin, then tap the upload/share icon in the upper right-hand corner, then tap Routing Apps. Since I have Google Maps, Glympse, and Waze installed on my iPhone, I see options to route with any of those three. Tapping one launches the respective app with the destination plugged in.
It's a few additional taps but at least it saves one from the more laborious effort of copying and pasting addresses into Google Maps manually. I too am disappointed in Apple for not allowing a user to select their own default apps for mapping, Siri searches, etc. To date I've not seen a logical explanation as to why it must be that way, so I continue to assume it's a proprietary lock-in manuever for no other reason than trapping consumers in Apple's web, perhaps for the analytics captured and subsequent marketing value of that information. In all fairness, I suspect Google does the same on Android.
In the case of mapping, the extra fumbling around to bring up a route in a more reliable mapping app while operating a motor vehicle introduces safety hazards which could be avoided by Apple cutting out the nonsense and allowing users to open map links directly in the mapping app of their choice. I'm not advocating operating a smartphone while driving but it does happen every day and will continue to happen. People will want new/altered driving directions while in motion and many roadways (eg. freeways) do not lend the convenience of pulling over to fidget with all of these excessive tap-throughs to get reliable guidance.