[vp8989]

"Whenever I investigate examples, though, I find the devs have to go through a ton of effort to avoid memory allocations"

Yep, also the median dev in a GC'ed language is simply incapable of writing super efficient code in these languages because they rarely have to. You would have to bring in the best of the best people from those communities or put your existing devs through a pretty significant education process that is similar in difficulty to just learning/using Rust.

The resulting code will be very different to what typical code looks like in those languages, so the supposed homogeneity benefits of just writing fast C#/Java when it's needed are probably not quite true. You'd basically have to keep that project staffed up with these kinds of people and ensure they have very good Prod observability to ensure regressions don't appear.

[sreque]

Yes, and I think one important aspect to this is the necessary CI/CD changes needed to support these kinds of optimizations. If your performance targets are tight enough that you are making significant non-standard optimizations in your GC'ed language, you're probably going to want some automated performance regression testing in your deployment pipeline to ensure you don't ship something that falls down under load. In my experience, building and maintaining those pipeline components is not easy.