You can't skip "spending a ton of time hardening one cluster" anyways. Having multiple clusters may help reduce the blast radius of _certain_ attacks, to some

You can't skip "spending a ton of time hardening one cluster" anyways. Having multiple clusters may help reduce the blast radius of _certain_ attacks, to some degree. However, managing multiple clusters is a lot more difficult than managing one, and you will potentially replicate bad practices, vulnerabilities to multiple places and increase maintenance burden.