That argument seems akin to avoiding C# because only MS supports it, or avoiding Python because CPython is the only serious implementation.
The language with the most sensible implementations seems to be javascript, I guess.
The language with the most sensible implementations seems to be javascript, I guess.
That said, Microsoft has very deep pockets, shows every indication of continuing to promote and use .NET extensively, and has also built up a lot of commercial buy-in over the 15 years or so that .NET has existed, which could carry it forward a while or fuel alternate implementations if something catastrophic happened.
PyPy is a serious implementation of Python. There are others and have been others over the years. While the corporate buy-in isn't the same as .NET or Java, there is a large amount of open source funding and buy-in from very diverse sources. Python is older than .NET as well.
Maybe the risk of using Rust is acceptable even if a bit larger, or the added risk is offset by Rust's good qualities - nonetheless, it's a very new language with no Microsoft-scale company behind it. Not precisely comparable.
It doesn't matter if Rust is perceived as having less ecosystem risk than Python or .NET, if in (say) 10 years it doesn't make significant headway against C, which is what it actually has to compete with. Here we could consider the examples of Pascal and Lisp - they competed with C, were arguably much better languages, and they lost anyway.
It's something worth considering if your dreams are founded on Rust.
Or avoiding Flash because only Adobe supports it, or SilverLight/VB because only MS supports it, or Objective C because only Apple supports it.