It's not that simple: Google makes money off of other services, a fair fraction of which is from Android users. If an Android user has a phone which is >3 years old, they are still likely to be buying apps through the Android store, using the phone to access Google's paid services, or generating data which Google uses for their ad sales.

The underlying problem with Android is that they're competing with Apple, where all of those sources generate ongoing revenue from older hardware devices, but haven't found an effective way to share revenue between the different parties involved to pay for long-term support. Apple has no problem shipping iOS updates because they don't need you to buy a new phone nearly as much if you're subscribing to iCloud, using Apple Music, and buying from the App Store.