Actually, it's more like Transit and hardly anything else. M-B has to be really worried, because everyone knows Sprinters rust like mad and can be expensive to maintain. Plenty bought Sprinters because, until lately, there was nothing else in the marketplace like it.
the updated 2014+ Sprinters (apparently, according to my digging) took many steps to address the rusting issues of the early models. Time will tell. The 2014 we ordered several months ago came with extensive rust proofing applied liberally right from the factory. I have faith in the 2.1L diesel. Not only does it get smoking fuel economy, but its a widely used power train used in the E class (volume seller) and other car models in the line up. Its clearly a tried and true power train, and with so many out there, I'm confident M-B engineered it right.
The fleet guys I talk to aren't balking so much at the "maintenance" costs, its that in many parts of the US, there aren't any M-B dealers for 100 miles and the close ones that are near them, aren't "equipped to work on Sprinters", and when you need parts, they have to wait a week+ for parts to come from Germany. downtime=bad. Not ideal if you are a business with a fleet of trucks.
M-B is bringing out a smaller cheaper version of the Sprinter this summer with the same 2.1 drivetrain. The Metris (V-class everywhere else), I can see this one as a bigger seller and the new fuel mileage champ.
The 'triple threat' build of the ProMaster just scares the bejeezus out of me. Built in Mexico, Chrysler parts, Fiat build quality. 'nuf said.
As for the Transit, the fleet guys are salivating over this because the top line engine 3.5EcoB V6 (while achieving virtually identical power and torque) gets 46% better fuel economy than the premium engine option V10 in the econoline, and 26+% better fuel economy than the V8's. Ask any fleet guy and he will tell you that the E series are bullet proof. We expect no less with the Transit. The 3.5Eco engine has been out since 2011, with 40+% of F150 owners going with this premium engine. I don't need to tell you how many F150s Ford moves in a year, so that equals a hell of a lot of engines! and its a tried, true and reliable power train.
Couple this with the fact that the Transit is around $10grand cheaper than a Sprinter, has cheap and easy accessibility to parts, wide dealer network, and should be built as well as the Econoline, its no surprise sales escalating.
I'll let you know how the Transit is, when ours (finally) arrives in April.