It's not normal to have to write new code to retain compatibility. ES6 was a huge change, but was sufficiently compatible with ES5 that you didn't need to doubl

It's not normal to have to write new code to retain compatibility. ES6 was a huge change, but was sufficiently compatible with ES5 that you didn't need to double your test matrix. C++11 is another example of a major evolution that retained backwards compatibility. There have been other painful upgrades (Swift, Rust) but these have been in younger languages with less established codebases; both have since shifted to a mature-language model. Python3 is unique in attempting to aggressively evolve a very mature language with deep established codebases, and has paid a heavy price.