GM certainly let the old 6.0 get stale over the years.
In my opinion GM got lazy and decided they didn't want to de-content and lower the compression of the 6.2 just to stick in the HD's when in hindsight they were developing the all new iron block 6.6 gas engine for the HD's.
Fleet customers certainly always request simplicity, easy maintenance, and longevity and the old 6.0 had that in spades. despite being long in tooth, if you follow TFLtruck, the old 6.0 was still competitive when compared to the offerings from ford and FCA.
With that being said the new 6.6 should be a welcome change at 401 HP and 464 ft/lbs tq, I guess we shall see.