Their first cut at "working on it" is to require that we pay Signal to store our backups for us (45 days of media and 100MiB total is not a useful free tier; I have more than 1 GiB of messages/media spanning years), when that's an entirely unnecessary restriction.

I don't know what you do for a living but it's very common when writing and releasing software to do it in phases. Earlier phases have a restricted feature set and feedback from the field/customers/users experiencing earlier phases informs choices in later phases.

Unless you have direct insights into their dev process, your claim that the restriction be "entitely unnecessary" seems overly strong.