I'm not being argumentative; I was genuinely trying to understand why you would aim to recreate Photoshop in particular, because Photoshop includes such a huge range of functionality. If it's about recreating the brush engine, that's pretty achievable. I would be curious to learn why Krita is not enough though.

Context: I've developed multiple Photoshop file format parsers/writers and know Photoshop at a very deep level. And would never want to replicate the specific set of tradeoffs Adobe made in the 1980s.