limit their size or the volume allowed per user
Fedi software like Misskey already did exactly that. Each user have limited “drive” which can be upgraded/customized per user. People even reuse image they already have on their drive, so it won’t be any duplicate files.
Regarding the interface: Krita is heavily designed to match artist workflow around the world. Here’s the complexity:
Different region, ages, and level of professionality have entirely different of common/standard app.
A lot of comic artist or illustrator that delves in anime/manga/ACG style in Asia use Clip Studio Paint as their main software. People from Western gaming industry often used Photoshop or Procreate. Teenager in Asia often use ibisPaint. PaintToolSAI for lightweight and customizable interface. There’s Rebelle for replicating traditional realistic painting.
Krita tried to cater to all of these people, not just “Photoshop users” that mostly just for image editing.