Hmm. Kind of makes you wonder why you're using Ruby at all, which is slow as hell. It appears that once they port all Rails libraries to Rust, it may not take that much effort to create Rust on Rails and get rid of Ruby.
Ruby is phenomenally easy to write. This post essentially describes a way to make Ruby less slow, which would reduce the incentive to leave the language.
Once they port all Rails libraries to Rust, you can still use the extremely-ergonomic Ruby programming language, but it won't be "slow as hell" anymore. And ruby performance is "tolerable in most circumstances" as-is.
I certainly agree that it could be awesome to have Rust on Rails for those who would find even Rails on Rust performance intolerable, and Iron's abstractions inadequate.
I find a great answer in this area is Crystal. I consider it 'go in ruby clothing'. Like go, Crystal is a typed language and outputs binary executables. They kept the beautiful ruby syntax and made better internals. http://crystal-lang.org/
Plenty of gems use native C to provide best possible performance. This doesn't mean everyone is planning to rewrite their Ruby applications in C.
Rust provides a safer option - which is worthwhile. There have been several examples of gems with memory issues. It doesn't change anything, unless this is a general "Ruby is too slow" sort of rant.
It does no such thing as make me wonder why I'm using Ruby. I can and do write Rust, and I enjoy doing so. But I write Ruby more quickly and can more effectively build tools for both my own consumption and that of others, using techniques that I can't use in Rust. Being able to use Rust or something that isn't an unpleasant minefield (lookin' at you, C) where I need it for performance is extremely valuable, but there's no reason to throw out the beneficial, powerful aspects of Ruby to do it.
How many platforms and frameworks have been completely ported over to a better language? There are many languages that have arguably succeeded PHP, and yet WordPress and Facebook remain on PHP, which would belie your assumption that "it may not take that much effort to [re]create [some framework] and get rid of [that framework's original language]"
Once they port all Rails libraries to Rust, you can still use the extremely-ergonomic Ruby programming language, but it won't be "slow as hell" anymore. And ruby performance is "tolerable in most circumstances" as-is.
I certainly agree that it could be awesome to have Rust on Rails for those who would find even Rails on Rust performance intolerable, and Iron's abstractions inadequate.