They already do, all they gotta do is take out the redirect to App popup and voila.

It's not amazing, just like most web apps from big tech.

It's not amazing by choice. Web apps for internet video playing would work fine, companies just make their mobile websites hostile as possible to push you towards their mobile app where you can get the same experience but with significantly more data collection on the app's part.

I wonder what kind of impact this would have on their ability to actually provide good ML recommendations. Based on reporting that would be a lot of data to give up

The reporting I've seen is extremely dramatic about the data being collected. The most useful thing to the recommendation algorithm is interactions: number of loops before scroll, like, comment, share, etc.

Which they have but that can just be blocked as well. Sure you will be able to access it with workarounds but most people won’t.

This was the original vision for iPhone, if I’m not mistaken

Hot take:

TikToks appeal is that it's digital heroin. People aren't going there because it's genuinely amazing stuff. People are going there because it's a dopamine hit. When you make it harder to view like having a web app that you have to refresh or click or have less smooth touch interactions, it'll get less engagement.

Edit: it will be more difficult to track user behavior and location with a web app.