With respect to point (1) you're neglecting that you actually have to make an investment into the ecosystem. You can't really go somewhere else that easily because you've paid for all the apps you use -- and all the data contained therein. This isn't possible or easy to port, and is therefore forfeit. This could be a few extra hundred or thousands of dollars.

With respect to point (2) nobody's saying you can't continue to shop exclusively in the walled garden. On macOS you can opt for the walled garden via App Store, or download from anywhere else. Being able to download from anywhere else doesn't devalue or detract from thew walled garden. If you like your little nook you can keep it.