About 6 years ago, Cummins was also working on a V6 and V8 for Chrysler, but those went away as well, due to various reasons - like Daimler running Chrysler into the ground, and Dodge's heavy truck division fighting against anything that would take away 5.9 sales.
On the subject of Duramax reliability, I've seen accounts of owners getting 200K miles or better out of the original injectors, by doing two simple mods -
Adding a lift pump, and switching to a 2 micron fuel filter.
Recently I sent the injectors out of my 87 300D to Pete Baranko of DBW LLC to have them rebuilt, and had opportunity to talk with him by phone. The main line of discussion ended up being about my 05 Jeep Liberty CRD, which has a Bosch CP3 HPCR system similar to that found on a Cummins and Duramax of the same time frame.
He said that my particular CP3 pump and solenoid injectors were comparatively simple and rugged, compared to the later piezo setups for post-2007 emissions. He added that with the mods I'd done early on - adding a lift pump, 30 micron primary and Cat 2 micron final fuel filters, return line fuel cooler - that I should easily see 300K to 350K miles out of the CP3 and injectors before they needed replacement!
Pete also explained part of the reason why adding a lift pump to a vacuum fuel system, such as was stock on the Liberty CRD and also on Duramax trucks, improved fuel economy - on my 2.8 L four banger, he said that with a vacuum fuel system the CP3 injection pump was probably taking upwards of 5 HP to keep the common rail supplied with high pressure fuel. By adding a 15 psi lift pump, I was significantly reducing the pumping losses on the CP3 and cutting it's HP requirements by probably close to half.