Once an android phone stops receiving updates, its only a matter of time until google play services stops working on that version of android, and then it becomes a big PITA to do anything google related with it, as it keeps redirecting away from web pages you to apps that tell you that you can't use them.
I wouldn't rely on Android to not go anywhere.
The new Android version might have introduced features that render that old app worthless.
Examples of such changes.
Locking down use of private APIs (abused by some apps), locking down NDK to the official Android native APIs and public shared objects, both cases app gets killed when doing naughty stuff, sandboxed file system (now really enforced in upcoming 11), permissions (now enforced with the Play Store minimum target version), and a couple more.
So if the app happens not to be constantly updated, it might just be rendered worthless with a new Android phone.
