Except many people doing exactly that and it working fine.
> backwards compatibility problems
Which aren't really a problem in practice. Definitely not worse then in any other language, yes there was some intentional breakage (soundness bug fixes) and unintentional breakage (compiler bugs) but most of this was in the very early stable rust versions since then accidental breakage which isn't caught and fixed before stable has become quite rare. As far as I know rust does more to find accidental breakage then many other languages.
> They always use $latest and what is in $latest changes so fast even if your rustc is 4 months old it already can't compile code written in $latest.
What is the problem with requiring a up-to date build chain? Slow long release cycles with a lot of patch back-porting is a software development model which for many (most?) projects has been deemed to not work well and has been replaced with something more in the direction of CI. Let's be honest even if we assume slow long releases are better for compilers, they still put quite an additional burden one the compiler development process which a is feasible if you have a lot of payed devs from large companies, but not really if only your core devs are payed (by the foundation and/or companies) but a lot of relevant contributions come from people not directly payed to work on rust.
> But right now it's painful and full of pitfalls.
I don't think so, but I guess we both can agree that thinks are improving, for me to make it even better and for you to maybe make it usable for the things you currently treat it as not suited for. ;-)