It looks like someone getting good at illustration. Older icons are far better illustrations. However icon design is not just about illustration, it's about clarity and affordances. Icons don't exist in isolation like an illustration, they exist alongside the rest of the UX and other app icons, and being recognisable is important.

All that to say, the sweet pot was likely somewhere in the middle of this timeline. The earliest icons aren't recognisable enough as they're too illustrative. The later icons aren't recognisable enough because they're too basic. The middle are pretty, clear from colour, clear from shape, well branded.