I'm pretty confident Simon Peyton-Jones would never find it acceptable that adding a new type class instance changes the behavior of existing code, regardless of what convenience it brings. Yes, the dot operator is useful (as SPJ acknowledges in that talk), but convenience at the cost of sacrificing the ability to reason about code and how its meaning evolves is not worth it to the typical Haskeller. SPJ isn't talking about Rust specifically in that talk; he's only talking about the convenience of IDE integration and namespacing based on the receiver.