One nice thing about the limited palette of tools we're generally willing to allow ourselves in the base libraries is that it steers us towards a pretty reasonable middle ground.
Since MPTCs and fundeps are off the table for the forseeable future in terms of Prelude library design, we're more or less forced into the subset of Haskell that works and infers nicely. Is what Foldable/Traversable allow us to build strictly the most general thing that could be built? No -- but it does tend to run a pretty good balance between rigorous laws, flexibility, ease of understanding, and inference.
When I see these pie in the sky options about generalizing everything away almost universally those rely on extensions that frankly don't infer well.
Foldable and Traversable do not rise to that level of complexity and can be implemented entirely within a framework we've had fully fleshed out for 15 years.
When is it enough to just integrate an improvement and move on? Should we try again at 30 years or do you think someone might have accidentally written another book in the meantime?
-Edward