My personal theory is that Helm may be ok for distributing a pre-packaged solution to other people. Then people mistook it for a tool that should be used in-house to deploy a company’s own systems, where it makes much less sense.

It makes absolute sense. You can use no variables and still deploy helm chart. It is a directory of plain old yaml objects. And add customization when you need as you evolve. Good luck doing that with kustomize.