I don't know that there is much question of replacing Rust when it isn't in use except as an unstable experiment with a lot of interest. You could replace Rust with outright vaporware.
Rust is not vaporware under any reasonable definition of that word; there is plenty of Rust there to run. It is not stable yet, to be sure; but there are plenty of people happily using it, some even in production.
The person you replied to did not say that Rust is vaporware.
At worst, it says that Rust's toehold is tenuous enough that even vaporware could replace it. I don't agree with that, but I can understand the sentiment. Think "knocked back with a feather".
pekk isn't saying Rust is vaporware. The meaning of pekk's post seems to me to be "Since Rust is almost never used in production, outright vaporware would allow people to do the same thing they mostly do with Rust: not use it." In other words, Rust has not yet found a niche, so you can't supplant it in a niche it doesn't yet exist in.