Not quite. Suppose instead of a single version of serde there's now 46 versions like in https://crates.io/crates/parquet to be able to use instances derived in some other crate X you have to use the same version of serde. Now, how should a crate decide which versions of serde to support?

All 46 and all optional? Supporting that would be painful. Just the last one? crates.io is a cemetery full of dead crates, with this support strategy any handful of crates picked at random are not going to be serde-compatible with each other.

A better solution would be a better support for compile time reflection so serde doesn't have to exist in its current state, but that's got delayed (by big macro conspiracy :)