Ah ok. We have scripts for automatic updating (from the skeleton) here. Making those more friendly to run on single themes/plugins could be a nice project. 
BTW, when I say they have different config, I don’t mean we’re duplicating the config everywhere. We have a versioned lint configs package which includes shared configs for prettier, eslint, stylelint, and ember-template-lint. This is used by core and all themes/plugins. Upgrading a theme/plugin to the latest rules is normally just a case of bumping the lint-configs version in the package.json.