I don't know about your claims that the App View is the server that has to store everything. It's not clear what they do exactly, looks like they are just another soft-centralizing layer like a web2 backend that talks to the BGS (I'll start calling "relay" BGS because that's a much better name) and caches data for user consumption and also talks to users's PDSes on behalf of clients?

In any case given that to run a BGS today you need many terabytes of disk https://whtwnd.com/bnewbold.net/entries/Notes%20on%20Running%20a%20Full-Network%20atproto%20Relay%20(July%202024) that must mean it is storing all the data. So we have both BGS and AppViews requiring storing all the data? 

 AFAIU you can have an App View that uses a relay, but could also be one software package. conceptually for the purposes of this discussion it’s really one thing.