To handle 25k qps with Rails, assuming 500ms per request, you need 12k Ruby processes. That probably means you run on 200 boxes with 64 CPUs each. Maybe you get a little bit of oversubscribing of processes to CPUs due to I/O. Conservatively it's still 100 boxes.

To handle 25k qps with Rust, you can run 10 boxes. Maybe less.

Deployments are complicated (even when we wrap them in simple abstractions). A "rare" delay that impacts 1% of boxes during a deploy will become common on 100 boxes but still somewhat rare on only 10.

Running a single rust process per box allows a connection pooler to be in-process. It's more efficient, it supports postgres prepared statements, it's one less operational headache, etc.