Ecosystem effects are definitely important to C++'s dominance in HFT, but it's also a domain where a lot of the guarantees Rust offers just aren't all that relevant. From a security perspective most code always runs in sandboxes accessible to only a select few whitelisted IPs. True, you don't want a segfault while you're in the middle of sending an order to an exchange, but most of those are pretty easily smoked out in simulation testing.