Let's say it's 2030, and I want to host my own matrix server. I would first compile a list of all matrix server implementations, and filter out those with confirmed bugs, and only then choose one (at random or some other criterion). The ones with bugs introduced by some library written in Hare/C/Rust/C++/X language will never be popular enough to even end up on the starting list cause people don't want to use buggy software (at least if there are options and in sufficiently-large ecosystem, there are options). And you might say that those bugs will go unnoticed - yes, but no more unnoticed than any other bug introduced by faulty logic but written in, say, Rust.
Also:
> so the language _will_ get used by a non-insignificant number of people
Hare is fairly opinionated (e.g. it mainly targets FOSS operating systems) so saying that it _will_ be used _just_ as a result of being related to ddevault (a person with a following), is a _wild_ guess. It does not matter how big of a following the author has, if the language itself is bad, it will not get used, or at least, it will get forgotten soon enough.
>In a sufficiently-large ecosystem...
In a sufficiently-large ecosystem, performance/security-critical software will be created using a language that is used not because of the author's following… Again: if the language itself is bad, it will not get used, or at least, it will get forgotten soon enough.
You can write buggy code in _any_ language.