I think it depends on the app. Some apps have a lot more interaction with the platform than others. E.g. writing a camera app in Flutter is going to be like 80% platform native code. Writing e.g. a GitHub app is probably going to be 0%.

If it is 0% most likely is doable as mobile Web, no need for hybrid headaches.

Yeah in a lot of cases you definitely could. But well written mobile apps are still better than well written mobile web sites. Sometimes much better depending on the application.

E.g. I would not like to give up the Gmail app and use the web interface.