I'm absolutely not suggesting dropping support for RHEL 5 users and in general I too tend to be conservative. But since the boat has been rocked, we may as well talk about it.
They're not extremists, they're big businesses. And big businesses like to use things like NSS and internal crates.io mirrors - combining these two things won't work with musl. And, as has been mentioned, plugins won't work so you'll miss out on serde derive as well.
For me it's a recognition that we have to upgrade someday and that remaining on CentOS 5 imposes a non-zero maintenance burden, so why not do a compatibility-preserving upgrade now to benefit from an eased burden.