Using the example of JSON, one of the issues in the Java ecosystem is that there are 3-5 JSON libraries(GJSON, Jackson, Org.json, Genson, etc.) that are commonly used. Today there is a clear choice for Rust, but in 5 years it will probably be more fractured.

If you have one or more dependencies that also need to parse JSON, and they choose differently, you could have cargo building multiple JSON libraries and including parts of them in your application, which would increase the bloat of the resulting binary.

This is of course a problem with any language and not Rust specific, but in an ideal world there would be one best way for doing most things included in the standard library and everyone would just use those.