It’s an impoverished view of what an OS is. A great OS is also providing APIs that enable a cohesive desktop, integrations that enable apps, and other core services.
For example, iOS has a built-in API for managing calendar events. That means my third-party to-do list can show a calendar view with everything from my iOS-managed calendar. Similarly, iOS has an API that enables third-parties to build apps that interact with Apple Music - so you don’t need to use the default client. Another example is focus modes/contextual computing - the OS enables configuring various settings in an automatable way. Another service is the health-tracking database - all my health apps share a common view of data, so my nutrition app can see my weigh ins, calories burned, and glucose levels - all coming from different devices.
On macOS, it goes beyond being just a launcher by providing rich file system services - for example, features to automate working on directories as contents change. It provides integration points to enable providing actions/services that work on text fields. It lets you configure dictionaries that support right-click/force press to look up. It implements a rich eMacs-inspired shortcut system in text fields.
Windows used to have APIs and integrations to support showing calendar events in the clock area. It was going to have a unified tab system that let you have windows with tabs provided by other apps (Sets).
In other words, an OS isn’t just a launcher. It’s a system.