Earlier quoted context omitted.
You might/should rotate the leaf keys regularly, but root keys are a massive pain to rotate (when you change a root key, you need to reprovision the corresponding cert onto every machine, as they're the root of trust for your PKI). You can buy a new HSM every 3 years to rotate those root keys, which fits fairly well into most organisation's decommissioning cycles.
Root keys are more of a pain to swap out, but AD does a pretty good job of handling the grunt work of distributing them across the organization. And the leaf keys should live on HSMs as well. You might be able to get by with a hardcoded root key if you're willing to accept the tradeoff, but that situation isn't going to work for the rest.
Leaf keys on HSMs is interesting. Personally, I view HSM and TPM stored keys about having an immutable identity for the device. Leaf keys are usually a bit more complicated - in the environments I work in, leaf keys are often tied to a service, not the host. The short lived nature also reduces the impact of ex-filtration.
Anyway, yeah, depends on the deployment and the org.