Full agreement, and with the addition of xpk¹/xfd² as natural extensions to that extensibility too. I see things like xfd supporting xz¹, and I'm simultaneously amazed that it exists and happy that I don't need to do xz {,de}compression on 68k ;)
I guess we have the essence of something similar-ish with libarchive⁴, but nobody(including me) has pushed the extra mile to get file dialogs to support random compression and decompression formats.
Beyond OT: I didn't realise how much stuff was still going on at aminet, but I love love LOVE that people are still dropping new car sets for Geoff Crammond's F1GP.
¹ http://aminet.net/package/util/pack/xpk_User
² http://aminet.net/package/util/pack/xfdmaster