What pros? Professional file clerks?
Funny, professional digital cameras aren't judged by their ability to manipulate the organization of the files on the memory card.
This thread is about doing professional art on an iPad. Professional usage is based on the work that's done, not where or how that work is stored. (Yes, professionals may also have certain requirements for storage and organization of their work, but those are not universal requirements.)
"I want to organize/re-organize things my way" or, "I want to change file attributes" are not essential functions for many pros and non-pros. I long ago gave up on maintaining a topic-driven, tree-structured Documents folder in favor of Search. Nice thing about Search is that it finds mis-filed as well as properly filed data. I most certainly do, and have, used computers professionally for many decades, so I hope I do qualify as a "pro."
Further, on-device storage is so amateur - to be accessible in our multi-user/multi-device world, some sort of server-based storage is essential. Who cares if the new drawing you created on iPad is stored in its default location when, ultimately, it has to be transferred to a server of some sort - the organization on the server is what really counts, whether you're using iCloud Drive or logging into your corporate VPN. There's no shortage of tools for accessing and organizing data on those servers.