Is the overhead going to matter if your content to block anyway? Surely any threading etc constructs are tiny compared to wire time, etc.
95% of what I write isn't performance-critical or even, really, performance-relevant. I still choose Rust for the vast majority of projects for ergonomic and correctness reasons.
Swift comes closest with codable/decodable but it's often still lacking the ergonomics of serde, especially the attribute options per field
A lot of people come to rust for other reasons, like compile time safety checks etc...
But most importantly, the delta between the performance of block_on and not, versus block_on and Python are massively different. You can write inefficient rust and still have a huge win over Python.