That's a very large assumption, one I think is outright false considering the discussion I've read about this post. The issue is that apps are allowed to embed webviews to do malicious things.
No, it's a large assumption that users want to open a link in an in-app browser. Why would they? On the other hand, there are clear motives for the app to keep users in the app as much as possible. Because the in-app browser in general is a degraded experience, for example being logged out of habitual websites, the user won't navigate the web as usual, and will soon return to the app. It's nothing but a dark pattern.
Your opinion is in the minority, even among the very technical folks I've seen discussing this issue. It's entirely false to reclassify a feature many users want as a dark pattern.
Let me give you an example personally: I want my RSS feed reader to use an in-app browser. Because unless I am going to get significantly more in depth, I just want to see the page content, and then swipe or back out to where I was in my feed very quickly.
