Quoted from Don44:I really loved AC/DC when I was young and thought Bon Scott was great but I never thought they were any good with Brian Johnson. I was surprised Back in Black was any good but everything since then has been pretty lame. They lost that blues rock sound they had with Bon Scott and Brian Johnson has that screechy voice that is hard to listen to.
Bon wrote much of BnB, I think that's why the falloff happened after.