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.

Yes, if you are receiving malicious data from the exchange, getting p0wnd is the least of your concern.