Getting banned from the Play Store on day one wasn't a great look for them. Nothing are also a pretty small fish in the pond; it was likely a predictable future EU diktat that they were getting ahead of to give themselves some positive PR. Nothing didn't have anything to lose because in their UK base nobody uses iMessage at all.

Apple might quote 1bn users but thats only because iMessage is baked into their products. This is not a number of active monthly users. WhatApp has 2bn users who have actively chosen to download the app. a big difference.

Nothing (and Sunbird) coud never have gotten the idea to work at scale because of the amount of Macs required on the backend, Macs Apple could push a debilitating software update out to at any point.