In my limited knowledge...
The 4000 series was a groundbreaking setup for Deere, and it was not without problems. They fixed them pretty well, but I bet from a competitve standpoint they had to do a new series. You will still hear proponants of other colors compare their brand to the 4000 series with comments about how tiring the hst pedals were to operate, the front axle's aggressiveness, whatever.
The 10 series also brought some amazing features. I thing the e controls are fantastic.
But competition from Kubotas 30 series will mean that they need to up the ante again, and the huge torque increases as well as new clean air requirements will end up getting us some nice product.
Obviously, they are covering their bases by offering a curved loader arm setup like NH.