Congrats! It's always surprised me that Amazon or one of the other Android vendors didn't do this themselves to make porting to FireOS or other services-included Android distributions easier, but I hope you get funding from them for doing their job for them.
Everyone wants to be Google, and I bet most managers see lock-in to their platform as a good thing, and ease of porting to competitors a bad thing.
At least, if they feel big enough to gain initial traction...
Huawei is pushing their own thing, alternative to Firebase[1]; Amazon as well[2].
There's a dozen services that provide cloud-based services to send push notifications to various services. I think the incentive to gain subscription revenue was greater than the opportunity to make these partially redundant.
I'm not sure why it took so long for something like UnifiedPush to appear. It took a fair bit of debugging, but ultimately, it was mostly up to one dedicated individual (S1m) with Android experience.
Before UnifiedPush, there was OpenPush[3], but it never materialized.
