As for registration fees, it sounds like they should use authenticator instead of SMS... and stop requiring a phone number to sign up. That is why I left Signal (went with Matrix). I don't see why anyone would want to tie their Signal to a phone. If you value privacy, why would you do that?
Servers cost seems excessive as well. I don't believe you need that many servers, even if you served a boat load of requests.
As for bandwidth.. okay, that may be the case. I am not sure how you can get that cost down.
The details are there in this post, but I can offer a few guesses. Users may be using multiple devices. And the service has to deliver to all the linked devices before ejecting the message from its storage. The time limit for storing and waiting for linked devices to come online is about a month. With tens of millions of users, this could add up.
shrugs
[1]: It was a copy of this cat video: https://www.youtube.com/shorts/8Ud1Cr76j8s