So if we ignore the technical merits based on which append-only data structures were chosen in general, we don't necessarily have additional technical merits for any particular time append-only was chosen.
The reasons why append-only structures were chosen in general apply surprisingly often to any particular system that scales to large data, which would like to be robust to a wide variety of real world problems. You won't see it in looking at the data structure because the hard parts are abstracted away from you. But you'll see it if you try to reimplement the same system from scratch, then scale it up to production.
Don't forget the security implications. If only root can run the compression/deletion script, all the compromised user can do is try to write to theirs. If you get into writing other users' data, that sucks, but nothing is deleted or exfiltrated, and the log is baked in. Break into root, well good luck with that on any system.
The subtle point I read in GP is the historic correlation between storage systems and data structures. Your point is equally valid in that this correlation is not indicative of non-general applicability.
Both points need to be kept in mind imho in design.
The reasons why append-only structures were chosen in general apply surprisingly often to any particular system that scales to large data, which would like to be robust to a wide variety of real world problems. You won't see it in looking at the data structure because the hard parts are abstracted away from you. But you'll see it if you try to reimplement the same system from scratch, then scale it up to production.