The E-series is the dominant player in the full size van market. Depending on how you slice it (cut-away versions for cube vans and ambulances, or regular vans), they make up 50 to 79% of the market.
Local courier folks that switched to the Sprinter, weren’t long switching back to the Econoline and Savannah. Reliability wasn’t great, and the drivers weren’t taking to the underwhelming “performance” of the little diesel engines.