I think you mean employees spend half a year to know enough Rust to be comfortable. That's how long it took me and others on my team to learn Java at a level where we knew the idioms, patterns and enough of the libraries. All of us had significant experience in other OO programming langauges.

Learning a language in days is a fairytale, the fact that you're so cavalier about this point makes it seem that you're more interested in doing advocacy than having an honest discussion.

Take the amount of time that it takes to become comfortable in a given language, and now increase that by an order of magnitude: this is how long it will take you to become comfortable writing C++ unsupervised. The language simply has too many subtle footguns to allow a beginner to be unleashed on a codebase (compare Java, where the worst a beginner will do is write a tangled mess of inflexible classes, but it will still largely work). The massive quantities of pre-C++11 documentation out there aren't doing the language any good either. And colleges (with the exception of game development programs) aren't teaching C++ anymore (and where they are, they're teaching C++98/03), so writing anything new in C++ means you're either going to maintain this yourself in perpetuity (talk about job security), or you're going to commit to a long period of coaching a beginner on the language, or you're going to hire an experienced C++ programmer for a significant multiple of what a less specialized programmer would have cost you.

So in the context of a conversation about the costs of training and long-term maintainability, well, you should probably write all your stuff in Java, but if you can't, then you're better off selecting Rust over C++. I can teach Rust to anyone that knows Python, and once you know Rust you're at least familiar with the most useful subset of C++.

There are plenty of C++ programmers to hire from. I know because we are hiring both at experienced and junior levels. Furthermore, a C++ programmer is not significantly more expensive than other programmers according to SO salary surveys. An experienced programmer will demand a good salary no matter which language he uses.

I gather from this post that you seem to use a risky strategy for your development - let juniors loose on a problem and hope that Rust/Java will save them. Or is that just a straw man? In any case, all juniors should get coaching, CI with static analysis and code reviews are likewise highly recommended.

Finally, I can't understand how someone can recommend Rust over C++ for long-term maintainability with a straight face.

The first stable version of Rust was announced four months ago, let's talk again in 10 years and see how your code from 2015 is doing.

I've hired people to work on .net apps (C#) as well as hiring devs with no F# experience and this is simply not my experience at all. I hired someone to do a spike of a project in Rust, ditto. And just this last week I met with one of my consultants that just started using F# the week prior. His F# code isn't quite idiomatic but he's written ~1000 lines of usable code. I've personally investigated and made bugfixes to a system written in a language/platform in which I had no experience.

Therefore I think it's quite fine to say it takes a few days to learn a bit of a language, enough to start contributing. Enough that it's not "job security" to pick something that provides a real benefit.