Very good questions.
There is a current move to treat anyone over 30 with high viral loads, even if their ALT levels are normal. Partly because we now know some immune activity is happening in these patients even though their ALT is not raised. Also we now know that integration of the virus into the cell’s DNA is happening early on too and it is linked with liver cancer.
Finally, there is data showing that people with hep B are more likely to experience high levels of liver inflammation after age 30, which can be controlled by antiviral treatment. Here it’s seen more of a prevention of liver damage that may occur in the near future. Liver damage makes liver cancer more likely. This is basically what @Albasil808 was saying.
That is an alternate strategy, to wait until your ALTs are high before treating. But the drawback is that many people may not get regularly tested or the tests may fall into a period between ALTs going up (and you would miss it in the blood tests).
That’s true, but the other parts of the liver need to be healthy. You can’t cut out 80% of a cirrhotic liver and still be able to survive because it is already under stress. Also, the liver is a very vascularised organ (it has a lot of blood vessels) so liver cancer can spread easily and that’s where it’s deadly.
Hope that makes things a bit clearer,