Personally, I don't see a few permanent connections as a problem. My GrapheneOS phone is degoogled, and therefore apps such as Signal fall back to a WebSocket connection. Battery life is probably somewhat impacted, but I use too few apps to notice. And in any case, this is much better than allowing Google to stick its nose into my business.
Yeah I'm disappointed this isn't pointed out in the opening paragraph. It's fair to critique Google for convincing devs that fcm is the only option, and obviously iOS is designed for Apple to do whatever they want, regardless of the owner's wishes, but Android does have other, viable, options. iOS and Android aren't equally bad here.