I’d love for this to happen (and WhatsApp to die in a fire alongside FB) but one of the issues I can see is spam.
iMessage currently uses iOS serial numbers to authenticate devices during provisioning (when you first power on a device it’ll present its serial number & other parameters and exchange those for a set of client certificates). I assume they are rate-limiting traffic from each device to a reasonable amount to avoid massive, automated spam. If someone wants to do large-scale spam the only way is to get access to a lot of iOS devices to get the keys from; this is expensive (and also difficult since you need to get superuser access on the device).
On Android, none of that exists. Simulating any arbitrary Android device is trivial, so they will no longer be able to use the “device” aspect to limit spam. They can’t ban devices either because the spammer can quickly generate one with another serial number and start spamming again. There’s also no issue with getting root access on the device to retrieve the APK and reverse-engineer the protocol to make spambots (on iOS it’s not just a simple app - the iMessage logic is closely tied to the overall push notification mechanism).