Comment 3 for bug 1961396

I think the fix here, is to allow adding constraints when adding a model. If the constraints for arch is empty, then use the os.HostArch to set it as default. We won't have broken anything for homogeneous deployments and we'll have a more sensible architecture. Heterogeneous deployments are still in the same position they're always have been.

If the user wants to change it they can either specify it with add-model command or if nothing is deployed, you can always use juju set-model-constraints.