Regardless of how the chain stretches, it still does, and the tensioner is apparently unable to keep the chain tight.
It's something I've never been able to figure out. Why couldn't Mercedes engineer this properly so that the tensioner could effectively keep the chain tight? Also, why did they use plastic rail guides that break as they get old? These two issues are so systemic with these engines, they'd have to have their heads in the sand to not know about it.
It is such common knowledge among technicians about replacing these chains every 100,000-120,000 miles, like it's a part of the engine's normal maintenance.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".