All of Shopify is mission critical. The code that constitutes the Shopify monolith is mission critical for sure. The company won't function without it.
This is somewhat circular, but: when your team decides to rewrite a component in a more complicated or more obscure (at least at Shopify) language to increase throughput rather than spinning up more instances of the current solution, then you're talking about a component that's even more critical than the other critical components.
It's just common practice - in Ruby some popular gems that parse XML or JSON come with a C extension that does some performance heavy calculation. Most gems don't do this, but when performance is very important some do.
Us Rubyists see no problem with that, and we don't take this as some kind of incentive to leave Ruby, maybe even the opposite; knowing that you can do 99% of your app in Ruby, and if ever needed, easily fill in the 1% with Rust/Go/C++ is very reassuring imo.
How do you define mission critical?