You’re forgetting the most important thing it is to users: an app. An app that sends messages. Messages that can be received by Android devices because iMessage automatically sends over SMS.
An iMessage sent remains as an iMessage when received.
This might be true from a certain technical perspective, depending on what you mean by “an iMessage”, but it’s certainly not true from a user perspective. The user sends a message from the iMessage app and doesn’t care much whether it’s delivered by iMessage or SMS. Messages sent by iMessage are automatically degraded when sent over SMS if they contain media or use iMessage-specific features. Ergo a message is sent by iMessage and received by an Android device as an SMS message.
If all of the iMessage servers exploded right now, nothing at all would change in Apple to Android messaging because iMessage was never involved.