the "suicide door" extended cab went away due to new government side-impact "pole test" crash regulations. Government is going to soon start implementing rollover roof-strength regulations as well. Which is a good thing, because generally body-on-frame trucks and SUV's have suffered in this area (roof strength).
Having a true B-pillar there does a huge amount for strengthening the cab in both a rollover and severe side-impact. IF they wanted to keep the "suicide doors" while still maintaining equal crash/rollover rigidness, Im sure its structurally possible from an engineering standpoint, BUT the extra bracing and steel required probably would have put the trucks overweight. So putting a B-pillar back in there was the most cost-effective light-weight solution to meeting the new regulations.
Im all for the design change; a crew cab/regular cab is much safer in any crash/rollover to begin with...and now that the "extended cabs" (double cab) have a B-pillar, they are equally as safe.
Between standard ESC/TCS/rollover-mitigation (stabilitrak) on all the new trucks including duallys, standard side-impact/curtain airbags/belt-anchor-pretensioners/belt-retractor-pretensioners (yes, the 2014+ half tons and 2015+ HD's have TWO seatbelt pretensioners per seat, as far as I know thats a first in the automotive industry), and a much stronger cab/roof design on the 2015+ GM HD's, they are the safest trucks yet.
Ben