Then the obvious question is "why would I use this instead of something else over S3" (ex. rclone), to which I think the answer is ease of use (don't need to deal with AWS yourself, encryption/deduplication/compression handled for you, nice interface), which isn't everything to everyone but is certainly useful.
You need a remote service that keeps backup readonly. You’re not covering attack scenarios if you just use raw object storage from your client machine.
I'd classify that under "ease of use" - you can do it with S3 yourself (your post is a pretty good explanation of the how, from a quick skim), or you can just use tarsnap and not worry about it.
You can see from my post that doing that _properly_ is quite convoluted and requires a good deal of technical skills.
So it's not just ease of use. It's actual _functionality_ to me - getting from raw object storage to a fully working, attack-resistant backup strategy, is not trivial; hence, comparing tarsnap (or rsync.net, or borgbase, or whatever) to B2 or S3 makes little to no sense.
You _could_ compare it to crashplan or backblaze personal backup if you like, but IIRC those don't work for *nix systems, only for Win and Mac.
Learning to use some backup tool that does same things sounds way better than paying 10 times more for the storage cost that lasts.
There's also a service like rsync.net where you can just rsync to the destination and they do the versioning and so on for less than 10th of the cost of tarsnap.