I think there is a lot of valuable things in the engine part: data presentation, tools, filters, non-destructive editing, etc. I wish GIMP had a "narrow waist" at this level, like games are usually split between levels and scripts and the engine, or compilers are split between parsing / syntax / high-level concerns and code generation at the intermediate language representation (the part "below waist" is often LLVM).
It just isn't the foundation you'd build on if you valued these things, they only added non-destructive recently it's not like it was core from the start, there's many other projects you could build upon that have this at their core.
Krita or honestly even Blenders shader graph would be a better and more modern starting point if that's what you wanted and you'd have a more active base where you're going to get value downstream in optimizations and more filters on a timeline that isn't multiple decades.
