All you're actually describing is that there's way less pressure for apps on the Google side to be as high quality as the Apple stuff. (Given the existence of the store alternatives, even if Google did enforce higher quality, those apps have plenty of other options to get on user's phones.) And that's kinda par for the course for the level of marketing-savvy that Google exhibits.

> The ability to install 3rd party apps has effectively zero effect on how well Apple curates their official app store.

You're missing the forest for the trees. Apple offers a curated environment largely through the curation of the app store. That curated environment is massively valuable to the average user. That's the asset Apple is trying to protect.