Another imprecise analogy would be to see traits as operators.

You decide to define the operator "serde::serialize" for "MyType" but then your are stuck because you can't override or select different operators for "MyType" because only one can exists.

That's a regular yet not super common issue with traits (and this is not exclusive to Rust). It's quite irritating because you wouldn't expect this from languages with this degree of modularity.