we put 125K on our 2004.5 duramax and then passed it on to our son. And we picked up a 2015.5 GMC 2500 denali duramax. same config, cc/sb/4x4.
All in all we love the 2015 Denali. lots of nice creature comfort and features that weren't in the 2004.5, gets better unloaded highway mileage by about 2mpg, same in town and towing.
Son now has 160K on the old 2004.5 and we put 28K on the new denali in the first year.
Don't regret trading up at all, would do it again, but the 04.5 certainly did everything we wanted it to do and probably will for another 150K miles or so.
But things kinda lined up. I liked the new 2015 improvements, son's S-10 he got from us had turned 270K miles and was starting to need work, and we figured if we got a new truck now, it would probably last us till we didn't need one anymore.
as far as emissions stuff and DEF, so far in 28K miles it's not been an issue. And with the GM a DEF tank lasts about 6K for us even towing half the time.