Your second paragraph: I expect that this must be a microscopic part of their total source code footprint. Is the software used for (equity/futures/options) exchange connectivity? (Oh no, please don't tell me "crypto".) I cannot think of anything else that would warrant the investment in the 2020s.

Depends what you mean by microscopic. If the entire project is a traffic forwarder, proxy, firewall-like, dispatcher / router etc. then that's the heart of the project and having the performance benefits of Rust there is absolutely crucial.

It was not only crypto. We're also talking telecom nodes and there are a lot of different protocols and needs around them (I was surprised how many, and some telecoms are finally waking up to the 21st century and started utilizing proper containerization and virtual networks to enforce compartmentalization / isolation / security).

On the broader topic: nowadays I default to Golang due to its better productivity and shorter dev iteration cycles, for what it's worth. However, there are projects where picking Golang over Rust would be irresponsible. Not the majority of all projects out there, absolutely, but there is an important subset that cannot be ignored.

PERSONAL NOTE: Sadly most of us will never work on those, and I am trying hard lately to change that for myself. Sick of web dev, I started hating it so much that I can do it extremely efficiently and almost with my eyes closed but I still can't forever ignore my needs; we are creative creatures and forcing ourselves to be assembly line workers only leads to spiritual and mental health death.