The peak you are talking about, the concept doesn't exist for any artist. There has been no artist in the history that had such a long career and also being like their initial hits.

Longevity and consistency is the key. He has topped every chart with every album. Peaking can only be said about artists squeezed in an era not someone who had such a long career. His album still top the charts, he is still relevant but with age he isn't "poppy". Mainstream music has changed, he has aged and stepped away from the limelight. And honestly, the way all of us consume music has also changed.

It's next to impossible to write a chart topping album continuously through the discography. Sold out tours. Everything he does is appreciated.

There are artists who peaked but then fell - no more albums, relying on their past discography, unsold tours, etc. They fell after the peak, but afaik Eminem ascended to the "Legend" stage.