The Python and Ruby interpreters and runtimes are written in C. Turns out that compiled systems languages are good choices for implementing other programming languages.
Maybe because Deno folks are pragmatics and not fanatics.
Rust is faster than V8. I'm not sure if ALL Deno tooling is written in Rust, but it makes sense to write, say, Typescript => JavaScript converter in Rust if it's 10-100x faster.
All that tells you is that if you want a very fast Typescript => JavaScript converter then you too should use Rust (or Go or C++) and not JavaScript executed by V8.
But writing a web app?
You're free to use Rust for that as well but millions of people chose nodejs instead and those are the people that might choose Deno in the future.
And indeed, you can see as a great example from the discussion on this thread how they eventually arrived at relying on Rust for most steps in the tsc toolchain: https://github.com/denoland/deno/issues/5432
The conclusion reached there is as you have stated: Rust is simply an order of magnitude faster in most cases.
Rust is not well suited for the same things as javascript / typescript / jsx / etc.
Also javascript and typescript are not well suited for compilers and dev tooling.
It shouldn't be seen as a negative that Deno is using rust in places where it really shines, which is a totally different use case than what deno is typically used for.
Other than browser based applications why would you choose JavaScript/TypeScript etc over Rust?
Developer tooling is not complicated and there is plenty of existing tools written in JavaScript, so it seems weird when they rule out there own language as not good enough. It should make anyone think twice before using Deno server side if it is not good enough for themselves.
The parts used to make your car were likely delivered to the factory by a semi truck, and not by another car. Should you buy a semi truck to go to the mall then, since even the carmaker thinks that cars aren't good enough? Obviously not. Different requirements are a thing.
Typescript/Deno is very ergonomic and presents a lower barrier for entry for writing web servers. However, it's a relatively large and slow to start runtime which makes it less than ideal for developer tools.
You should use the right tool for the job. Not every language is good for every job or every person.
I have done work on compiler, developer tooling and a lot of other stuff people would consider 'compilated' and working on those was far less complex than some business logic in API's I have had to work upon.
So if the experienced developers writing Deno are using Rust for type safety etc to catch their mistakes, then why would you trust yourself to use a less strict lanaguage on a likely more complex project?
My guess: Deno would perform too slowly which would hurt the UX. It would also be harder to maintain than a language like Rust. And of course Rust doesn't need a runtime.
Not everything is about performance. Deno lets developers be more efficient than Rust, and it is easier to hire developers that already knows TypeScript/Node.