This is the Achilles’ heel of phone-based devices: the device will be left behind by newer phones and thus rendered useless. There are multimeters from the 70’s (and earlier!) in working use today, alas you will not find a Mooshimeter still usable in 2055 for want of a working phone to drive it.
Craig mentioned above to keep the APK – I’d go further: keep a working phone with the app on it. For example, my new phone is an Android 10 device and whilst the app works, it raises warnings about being out of date. I expect the app will stop working sometime over Android 10’s lifetime.