Different syntax = different ways of doing things = codebases that diverge in implementation. It doesn't matter if the new way doing things is conceptually simpler, you are now creating 2 ways to do things based on what version of the compiler you use, which people will have to know, therefore increasing complexity, not reducing it. I don't understand how people don't get this by now. Saying "oh just learn and use the latest version" is completely unrealistic guidance.

And while its fine for high level scripting languages like Python, because the whole idea is to abstract a lot of the functionality into the syntax, the issue with Rust is that its trying to be the next C and be applicable for writing critically important pieces of software like parts of the linux kernel. You absolutely must have a locked down language set for this. The reason C is still used for the kernel isn't because of circumstances, its specifically because there is very few ambiguities in the core language.