If you need a few hundred lines of ifdef in a codebase with millions of lines it is portable in the sense that you are able to port it with reasonable effort.
That's nice, you're welcome to continue writing your ifdefs. Nothing you or anyone else has said has built a compelling case for alternate frontends. You're saying "it's not much effort". Sure, but I prefer 0 effort.
What we're saying is that everyone actually using Rust today is fairly happy with how the language is. That's why Rust has had an 80%+ approval rating among Rust developers since its creation and C++ is at 49% (https://survey.stackoverflow.co/2023/#section-admired-and-de...).
This suggestion of alternate frontends still has no good technical reason driving it, just people pattern matching on their past experience. If it happens, all it will do is lower Rust's approval to C++'s level.